Presentation is loading. Please wait.

Presentation is loading. Please wait.

Principles of Electrical Currents

There are copies: 3
MENS and IONTOPHORESIS. MENS No universally accepted definition or protocol & has yet to be substantiated This form of modality is at the sub-sensory.

T.E.N.S. Trancutaneous Electrical Nerve Stimulation

Interferential Current - IFC

Similar presentations

Presentation on theme: "Principles of Electrical Currents"— Presentation transcript:

1 Principles of Electrical Currents

2 Electricity is an element of PT modalities most frightening and least understood.
Understanding the basis principles will later aid you in establishing treatment protocols.

3 General Therapeutic Uses of Electricity
Controlling acute and chronic pain Edema reduction Muscle spasm reduction Reducing joint contractures Minimizing disuse/ atrophy Facilitating tissue healing Strengthening muscle Facilitating fracture healing

4 Contraindications of Electrotherapy
Cardiac disability Pacemakers Pregnancy Menstruation (over abdomen, lumbar or pelvic region) Cancerous lesions Site of infection Exposed metal implants Nerve Sensitivity

5 Terms of electricity Electrical current: the flow of energy between two points Needs A driving force (voltage) some material which will conduct the electricity Amper: unit of measurement, the amount of current (amp) Conductors: Materials and tissues which allow free flow of energy

6 Fundamentals of Electricity
Electricity is the force created by an imbalance in the number of electrons at two points Negative pole an area of high electron concentration (Cathode) Positive pole and area of low electron concentration (Anode)

7 Charge An imbalance in energy. The charge of a solution has significance when attempting to “drive” medicinal drugs topically via inotophoresis and in attempting to artificially fires a denervated muscle

8 Charge: Factors to understand
Coulomb’s Law: Like charges repel, unlike charges attract Like charges repel allow the drug to be “driven” Reduce edema/blood

9 Charge: Factors Membranes rest at a “resting potential” which is an electrical balance of charges. This balance must be disrupted to achieve muscle firing Muscle depolarization is difficult to achieve with physical therapy modalities Nerve depolarization occurs very easily with PT modalities

10 Terms of electricity Insulators: materials and tissues which deter the passage of energy Semiconductors: both insulators and conductors. These materials will conduct better in one direction than the other Rate: How fast the energy travels. This depends on two factors: the voltage (the driving force) and the resistance.

11 Terms of electricity Voltage: electromotive force or potential difference between the two poles Voltage: an electromotive force, a driving force. Two modality classification are: Hi Volt: greater than V Lo Volt: less than V

12 Terms of electricity Resistance: the opposition to flow of current. Factors affecting resistance: Material composition Length (greater length yields greater resistance) Temperature (increased temperature, increase resistance)

13 Clinical application of Electricity: minimizing the resistance
Reduce the skin-electrode resistance Minimize air-electrode interface Keep electrode clean of oils, etc. Clean the skill on oils, etc. Use the shortest pathway for energy flow Use the largest electrode that will selectively stimulate the target tissues If resistance increases, more voltage will be needed to get the same current flow

14 Clinical application of Electricity: Temperature
Relationship An increase in temperature increases resistance to current flow Applicability Preheating the tx area may increase the comfort of the tx but also increases resistance and need for higher output intensities

15 Clinical Application of Electricity: Length of Circuit
Relationship: Greater the cross-sectional area of a path the less resistance to current flow Application: Nerves having a larger diameter are depolarized before nerves having smaller diameters

16 Clinical Application of Electricity: Material of Circuit
Not all of the body’s tissues conduct electrical current the same Excitable Tissues Nerves Muscle fibers blood cells cell membranes Non-excitable tissues Bone Cartilage Tendons Ligaments Current prefers to travel along excitable tissues

17 Stimulation Parameter:
Amplitude: the intensity of the current, the magnitude of the charge. The amplitude is associated with the depth of penetration. The deeper the penetration the more muscle fiber recruitment possible remember the all or none response and the Arndt-Schultz Principle

18 Simulation Parameter Pulse duration: the length of time the electrical flow is “on” also known as the pulse width. It is the time of 1 cycle to take place (will be both phases in a biphasic current) phase duration important factor in determining which tissue stimulated: if too short there will be no action potential

19 Stimulation Parameter:
Pulse rise time: the time to peak intensity of the pulse (ramp) rapid rising pulses cause nerve depolarization Slow rise: the nerve accommodates to stimulus and a action potential is not elicited Good for muscle reeducation with assisted contraction - ramping (shock of current is reduced)

20 Stimulation Parameters
Pulse Frequency: (PPS=Hertz) How many pulses occur in a unit of time Do not assume the lower the frequency the longer the pulse duration Low Frequency: 1K Hz and below (MENS .1-1K Hz), muscle stim units) Medium frequency: 1K ot 100K Hz (Interferential, Russian stim LVGS) High Frequency: above 100K Hz (TENS, HVGS, diathermies)

21 Stimulation Parameter:
Current types: alternating or Direct Current (AC or DC) AC indicates that the energy travels in a positive and negative direction. The wave form which occurs will be replicated on both sides of the isoelectric line DC indicated that the energy travels only in the positive or on in the negative direction DC AC

22 Stimulation Parameter:
Waveforms; the path of the energy. May be smooth (sine) spiked, square,, continuous etc. Method to direct current Peaked - sharper Sign - smoother

23 Stimulation Parameter:
Duty cycles: on-off time. May also be called inter-pulse interval which is the time between pulses. The more rest of “off” time, the less muscle fatigue will occur 1:1 Raito fatigues muscle rapidly 1:5 ratio less fatigue 1:7 no fatigue (passive muscle exercise)

24 Stimulation Parameter:
Average current (also called Root Mean Square) the “average” intensity Factors effective the average current: pulse amplitude pulse duration waveform (DC has more net charge over time thus causing a thermal effect. AC has a zero net charge (ZNC). The DC may have long term adverse physiological effects)

25 Stimulation Parameter:
Current Density The amount of charge per unit area. This is usually relative to the size of the electrode. Density will be greater with a small electrode, but also the small electrode offers more resistance.

26 Capacitance: The ability of tissue (or other material) to store electricity. For a given current intensity and pulse duration The higher the capacitance the longer before a response. Body tissues have different capacitance. From least to most: Nerve (will fire first, if healthy) Muscle fiber Muscle tissue

27 Capacitance: Increase intensity (with decrease pulse duration) is needed to stimulate tissues with a higher capacitance. Muscle membrane has 10x the capacitance of nerve

28 Factors effecting the clinical application of electricity
Factors effecting the clinical application of electricity Rise Time: the time to peak intensity The onset of stimulation must be rapid enough that tissue accommodation is prevented The lower the capacitance the less the charge can be stored If a stimulus is applied too slowly, it is dispersed

29 Factors effecting the clinical application of electricity
An increase in the diameter of a nerve decreased it’s capacitance and it will respond more quickly. Thus, large nerves will respond more quickly than small nerves. Denervated muscles will require a long rise time to allow accommodation of sensory nerves. Best source for denervated muscle stimulation is continuous current DC

30 Factors effecting the clinical application of electricity:
Ramp: A group of waveforms may be ramped (surge function) which is an increase of intensity over time. The rise time is of the specific waveform and is intrinsic to the machine.

31 Law of DuBois Reymond: The amplitude of the individual stimulus must be high enough so that depolarization of the membrane will occur. The rate of change of voltage must be sufficiently rapid so that accommodation does not occur The duration of the individual stimulus must be long enough so that the time course of the latent period (capacitance), action potential, and recovery can take place

32 Muscle Contractions Are described according to the pulse width
1 pps = twitch 10 pps = summation 25-30 pps = tetanus (most fibers will reach tetany by 50 pps)

33 Frequency selection: 100Hz - pain relief 50-60 Hz = muscle contraction
1-50 Hz = increased circulation The higher the frequency (Hz) the more quickly the muscle will fatigue

34 Electrodes used in clinical application of current:
Electrodes used in clinical application of current:At least two electrodes are required to complete the circuit The body becomes the conductor Monophasic application requires one negative electrode and one positive electrode The strongest stimulation is where the current exists the body Electrodes placed close together will give a superficial stimulation and be of high density

35 Electrodes used in clinical application of current:
Electrodes spaced far apart will penetrate more deeply with less current density Generally the larger the electrode the less density. If a large “dispersive” pad is creating muscle contractions there may be areas of high current concentration and other areas relatively inactive, thus functionally reducing the total size of the electrode A multitude of placement techniques may be used to create the clinical and physiological effects you desire

36 General E-Stim Parameters

37 E-Stim for Pain Control: typical Settings

38 High Volt Pulsed Stimulation

39 HVPS The application of monophasic current with a known polarity
typically a twin-peaked waveform duration of msec Wide variety of uses: muscle reeducation (requires 150V) nerve stimulation (requires 150V) edema reduction pain control

40 Clinical Application:
Physiological response can be excitatory and non-excitatory Excitatory Peripheral nerve stimulation for pain modulation (sensory, motor and pain fibers) Promote circulation: inhibits sympathetic nervous system activity, muscle pumping and endogenous vasodilatation Non-Excitatory (cellular level) Protein synthesis Mobilization of blood proteins Bacteriocyte affects (by increased CT micro-circulation there is a reabsorption of the interstitial fluids)

41 General Background Early in history HVS was called EGS, then HVGS, then HVPS Current qualifications to be considered HVS Must have twin peak monophasic current Must have 100 or 150 volts (up to 500 V)

42 HVPS Precautions Contraindications
Stimulation may cause unwanted tension on muscle fibers Muscle fatigue if insufficient duty cycle Improper electrodes can burn or irritate Intense stim may result in muscle spasm or soreness Contraindications Cardiac disability Pacemakers Pregnancy Menstruation Cancerous lesion Infection Metal implants Nerve sensitivity Indications past slide

43 Treatment Duration General - 15-30 minutes repeated as often as needed
Pain reduction - sensory 30 minutes with 30 minute rest between tx

44 Current Parameters greater than 100-150 V usually provides up to 500 V
high peak, low average current strength duration curve = short pulse duration required higher intensity for a response high peak intensities (watts) allow a deeper penetration with less superficial stimulation

45 Current Parameters Pulse Rate: Modulations Pulse Charge
ranges from pps varies according to the desire clinical application Current Pulse Charge related to an excess or deficiency of negatively charged particles associated with the beneficial or harmful responses (thermal, chemical, physical) Modulations intrapulse spacing duty cycle: reciprocal mode usually 1:1 ratio ramped or surged cycles Clinical Considerations: always reset intensity after use (safety) electrode arrangements may be mono or bipolar units usually have a hand held probe for local (point) stimulation most units have an intensity balance control

46 Application Techniques
Monopolar: 2 unequal sized electrodes. Smaller is generally over the treatment site and the large serves as a dispersive pad, usually located proximal to the treatment area Bipolar: two electrodes of equal size, both are over or near the treatment site Water immersion - used for irregularly shaped areas Probes: one hand-held active lead advantages: can locate and treat small triggers disadvantages: one on one treatment requires full attention of the trainer

47 Electrodes Material carbon impregnated silicone electrodes are recommended but will develop hot spots with repeated use you want conductive durable and flexible material tin with overlying sponge has a decreased conformity and reduced conductivity

48 Electrodes Size based on size of target area
current density is important. The smaller the electrode size the greater the density

49 Neuromuscular Stimulation
Roles: re-educate a muscle how to contract after immobilization (does not produce strength augmentation but retards atrophy)

50 Pain Control Roles: Control acute or chronic pain both sensory (gate control pps)) and motor level (opiate release - through voltage)

51 Pain Control - Opiate Release Setting

52 Control and Reduction of Edema
Roles: Sensory level used to limit acute edema Motor-level stimulation used to recude subacute or chronic inflammation

53 Motor-Level Edema Reduction
Cell Metabolism: increased and may increase blood flow Wound Healing: May increase collagnase levels and inhibit bacteria in infected wounds (for this effect 20 min - polarity followed by 40 min + polarity recommended)

54 T.E.N.S.

55 General Concepts: An Approach to pain control
Trancutaneous Electrical Nerve Stimulation: Any stimulation in which a current is applied across the skin to stimulate nerves 1965 Gate Control Theory created a great popularity of TENS TENS has 50-80% efficacy rate TENS stimulates afferent sensory fibers to elicit production of neurohumneral substances such as endorphins, enkephalins and serotonin (i.e. gate theory)

56 TENS Indications Precautions Control Chronic Pain
Management post-surgical pain Reduction of post-traumatic & acute pain Precautions Can mask underlying pain Burns or skin irritation prolonged use may result in muscle spasm/soreness caffeine intake may reduce effectiveness Narcotics decrease effectiveness

57 TENS may be: high voltage interferential acuscope
low voltage AC stimulator classical portable TENS unit

58 Biophysical Effects Primary use is to control pain through Gate Control Theory May produce muscle contractions Various methods High TENS (Activate A-delta fibers) Low TENS (release of -endorphins from pituitary) Brief-Intense TENS (noxious stimulation to active C fibers)

59 Techniques of TENS application:
Conventional or High Frequency Acupuncture or Low Frequency Brief Intense Burst Mode Modulated

60 Protocol for Various Methods of TENS

61 Conventional Tens/High Frequency TENS
Paresthesia is created without motor response A Beta filers are stimulated to SG enkephlin interneuron (pure gate theory) Creates the fastest relief of all techniques Applied 30 minutes to 24 hours relief is short lives (45 sec 1/2 life) May stop the pain-spasms cycle

62 Application of High TENS
Pulse rate: high Hz (generally 80), constant Pulse width: narrow, less than 300 mSec generally 60 microSec Intensity: comfortable to tolerance

63 Set up: 2 to 4 electrodes, often will be placed on post-op. Readjust parameters after response has been established. Turn on the intensity to a strong stimulation. Increase the pulse width and ask if the stimulation is getting wider (if deeper=good, if stronger...use shorter width)

64 Low Frequency/Acupuncture-like TENS:
Level III pain relief, A delta fibers get Beta endorphins Longer lasting pain relief but slower to start Application pulse rate low 1-5ppx (below 10) Pulse width: microSec Intensity: strong you want rhythmical contractions within the patient’s tolerance

65 Burst Mode TENS Carrier frequency is at a certain rate with a built in duty cycle Similar to low frequency TENS Carrier frequency of Hz packaged in bursts of about 7 bursts per second Pulses within burst can vary Burst frequency is 1-5 bursts per second Strong contraction at lower frequencies Combines efficacy of low rate TENS with the comfort of conventional TENS

66 Burst Mode TENS - Application
Pulse width: high microSec Pulse rate: pps modulated to 1-5 burst/sec Intensity: strong but comfortable treatment length: minutes

67 Brief, Intense TENS: hyper-stimulation analgesia
Stimulates C fibers for level II pain control (PAG etc.) Similar to high frequency TENS Highest rate (100 Hz), 200 mSec pulse width intensity to a very strong but tolerable level Treatment time is only 15 minutes, if no relief then treat again after 2-3 minutes Mono or biphasic current give a “bee sting” sensation Utilize motor, trigger or acupuncture points.

68 Brief Intense TENS - Application
Pulse width: as high as possible Pulse rate: depends on the type of stimulator Intensity: as high as tolerated Duration: 15 minutes with conventional TENS unit. Locus stimulator is advocated for this treatment type, treatment time is 30 seconds per point.

69 Locus point stimulator
Locus (point) stimulators treatment occurs once per day generally 8 points per session Auricular points are often utilized Treat distal to proximal Allow three treatment trails before efficacy is determined Use first then try other modalities

70 Modulated Stimulation:
Keeps tissues reactive so no accommodation occurs Simultaneous modulation of amplitude and pulse width As amplitude is decreased, pulse width is automatically increased to deliver more consistent energy per pulse Rate can also be modulated

71 Electrode Placement: May be over the painful sites, dermatomes, myotomes, trigger points, acupuncture points or spinal nerve roots. May be crossed or uncrossed (horizontal or vertical

72 Contraindications: Demand pacemakers over carotid sinuses Pregnancy
Cerebral vascular disorders (stroke patients) Over the chest if patient has any cardiac condition

73 Interferential Current - IFC

74 Interferential Current
History: In 1950 Nemec used interference of electrical currents to achieve therapeutic benefits. Further research and refinements have led to the current IFC available today Two AC are generated on separate channels (one channel produces a constant high frequency sine wave ( Hz) and the other a variable sine wave The channels combine/interface to produce a frequency of Hz (medium frequency)

75 Effects of IFC treatment:
Sensory nerve fibers - Pain reduction - receive a lower amplitude stimulation than the area of tissue affected by the vector, thus IFC is said to be more comfortable than equal amplitudes delivered by conventional means Muscle fatigue - muscle spasm - is reduced when using IFC versus HVS due to the asynchronous firing of the motor units being stimulated

76 Positive effects of IFC include:
reduction of pain and muscle discomfort following joint or muscle trauma these effects can be obtained with the of IFC and without associated muscle fatigue which may predispose the athlete to further injury.

77 Principles of wave interference - Combined Effects
Constructive, Destructive, & Continuous Constructive interference: when two sinusoidal waves that are exactly in phase or one, two, three or more wavelengths our of phase, the waves supplement each other in constructive interference = +

78 Principles of wave interference - Combined Effects
Destructive interference: when the two waves are different by 1/2 a wavelength (of any multiple) the result is cancellation of both waves = +

79 Principles of wave interference - Combined Effects
Continuous Interference Two waves slightly out of phase collide and form a single wave with progressively increasing and decreasing amplitude = +

80 Amplitude-Modulated Beats:
Rate at which the resultant waveform (from continuous interference) changes When sine waves from two similar sources have different frequencies are out of phase and blend (heterodyne) to produce the interference beating effect

81 IFC Duration of tx 15-20 minutes Precautions Contraindications
Burst mode typically applied 3x a week in 30 minute bouts Precautions same as all electrical currents Contraindications Pain of central origin Pain of unknown origin Indications Acute pain Chronic pain Muscle spasm

82 IFC Techniques of treatment:
Almost exclusively IFC is delivered using the four-pad or quad-polar technique. Various electrode positioning techniques are employed: Electrodes (Nemectrody: vacuum electrodes): four independent pads allow specific placement of pads to achieve desired effect an understanding of the current interference is essential four electrodes in one applicator allows IFC treatment to very small surface areas. The field vector is pre-determined by the equipment

83 Quad-polar Technique Pads placed at 45º angles from center of tx area
Can reduce inaccuracy of appropriate tissues by selecting rotation or scan Channel B Channel B Channel A SCAN Channel A

84 Bipolar Electrode Placement
The mix of two channels occurs in generator instead of tissues Biopolar does not penetrate tissues as deeply, but is more accurate When effects are targeted for one muscle or muscle group only one channel is used

85 Two-circuit IFC: At other points along the time axes the wave amplitude will be zero because the positive phase from one circuit cancels the negative phase from the second circuit (destructive interference) The rhythmical rise and fall of the amplitude results in a beat frequency and is equal to the number of times each second that the current amplitude increases to its maximum value and then decreases to its minimum value

86 Special Modulations of IFC:
Constant beat frequencies (model): the difference between the frequencies of the two circuits is constant and the result is a constant beat frequency. That is, if the difference in frequency between the two circuits is 40 pps, the beat frequency will be constant at 40 bps.

87 Special Modulations of IFC:
Variable beat mode: the frequency between the two circuits varies within preselected ranges. The time taken to vary the beat frequency through any programmed range is usually fixed by the device at about 15 sec. IFC machines often allow the clinician to choose from a variety of beat frequency programs.

88 Pain Control Similar to TENS - beat frequency 100Hz
Low beat frequencies when combined with motor level intensities (2-10Hz) initiate the release of opiates 30 Hz frequencies affects the widest range of receptors

89 Neuromuscular Stimulation
Beat frequency of approximately 15 HZ is used to reduce edema General Parameters

90 IFC Technique of treatment:
Electrode placement: The resultant vector should be visualized in placing the electrodes for a treatment . The target tissue should be identified and the vector positioned to hit that area. Typically at 45º angles is most effective. Segregation of the pin tips is essential in the proper electrode positioning for IFC. The electrodes may be of the same size or two different sizes (causing a shift in the intersecting vector). Treatment through a joint has also been advocated without adequate research to establish efficacy of the treatment technique.

91 Bone Stimulating Current:
Bone Stimulating Current:Bone Stimulating Current:IFC has been used (Laabs et al) studied the healing of a surgically induced fracture in the forelegs of sheep. Their study indicated an acceleration of healing in the sheep treated with IFC as compared to the control group

92 Bone Stimulating Current:
This study validated an earlier study by Gittler and Kleditzsch which showed similar results in callus formation in rabbits. Several other studies have shown an increase in the healing rate of fractures but the exact mechanism by which the healing occurs is not understood.

93 Bone Stimulating Current:
Some speculation is that an increased blood flow to the injured area is produced which allowed natural healing processes to occur more rapidly. In one study (mandible fractures ) the IFC caused very mild muscle contraction of the jaw and this muscle activity was thought to have been a potential accelerator of the healing.


95 MENS No universally accepted definition or protocol & has yet to be substantiated This form of modality is at the sub-sensory or very low sensory level current less than 1000A (approx 1/1000 amp of TENS)

96 Biophysical Effects Theory:
Currents below 500A increases the level of ATP (high Amp decreases ATP levels) Increase in ATP encourages amino acid transport and increased protein synthesis MENS reestablishes the body’s natural electrical balance allowing metabolic energy for healing without shocking the system (other types of e-stim)

97 MENS Duration Precautions Contraindications Indications
30 min to 2 hours up to 4x a day Precautions Dehydrated patients on Scar tissue (too much impedance) Contraindications Pain of unknown origin Osteomyelitis Indications Acute & Chronic Pain Acute & Chronic Inflammation Edema reduction sprains & Strains Contusion TMJ dysfunction Neuropathies Superficial wound healing Carpal Tunnel Syndrome

98 Electrode Placement Electrodes should be placed in a like that transects the target tissues Remember that electrical current travels in path of least resistance, thus it is not always a straight line. TARGET

99 Application Techniques
Standard electrical stimulation pads generator may have bells & Whistles since MENS is subsensory Probe

100 Bone Stimulating Current:
MENS Has been advocated in the healing of bone, using implanted electrodes and delivering a DC current with the negative pole at the fracture site. Further use of MENS has allowed increased rate of fracture healing using surface electrodes in a non-invasive technique. Theories on the physiology behind the healing focus on the electrical charge present in the normal tissue as compared to the electrical charge found with the injured tissue. MENS is said to allow an induction of an electrical charge to return to he tissues to a better “healing” environment

101 Iontophoresis

102 Iontophoresis: The transfer of ions across the skin (transdermal)by use of continuous direct current Iontophoresis is based on the principle that an electrically charged electrode will repel a similarly charged ion (first reported by LeDuc in 1903). Delivers a low-volt High-amp DC current Local blood flow is increased for 1 hour post tx

103 Iontophoresis Duration of Tx: Indications Contraindications
Based on intensity desired usually every other day for 3 weeks Indications Acute or Chronic Inflam Arthritis Myositis Myofacial Pain Syndromes Invasive method for delivering drugs Contraindications Hypersensitivity to electrical currents Contraindications to meds. Pain of unknown origin Precautions Prescription Dosage Do not reuse electrode Burns if intensity to great

104 Iontophoresis Effects of treatment depends on the ion(s) delivered
musculoskeletal inflammatory conditions (tendonitis, bursitis) have been successfully treated: Using desamethosone sodium phosphate (decadron) and Xylocaine Reduction of edema has been achieved by driving hyaluronidase Transitory (5min) local anesthesia has been produced by delivering lidocaine to the tissues. The anesthesia was better than that achieved by topical application but less effective than infiltration of the area with lidocaine.

105 Current Amp (mA) x Tx Duration - mA/min
Medication Dosage Medication dose delivered during tx is measured in mA based on relationship of amperage, tx duration Current Amp (mA) x Tx Duration - mA/min Iontophoresors are dose-oriented - where user indicated desired tx does and generator calculated duration and intensity

106 Biophysical Effects Dependant on Medication See following chart

107 Sample Medications

108 Electrode Placement Delivery Electrode (drug electrode)
placed over target tissue Active electrode (dispersive electrode) place 4-6 inches from drug electrode

109 Side Effects: Tissue “burning”
An alkaline reaction occurs under the cathode (negative electrode) which is much more caustic to the skin than the acidic reaction occurring at the anode. The cathode may be increased in size to attempt to decrease this caustic reaction

110 Side Effects: Tissue “burning”
Continuous unidirectional current (as needed for iontophoresis) tends to cause tissue irritation because skin will not tolerate current density greater than 1mA/ Thin tissue areas, areas of skin abrasion and areas of scarring are certain areas to avoid. This potential for burn is exacerbated by the fact that there is an anesthetic effect of DC under the electrode. Thus tissue irritation may develop without the patient’s realization Don’t need to drive every day 1-2x a week

Download ppt "Principles of Electrical Currents"

Similar presentations

Ads by Google