Direct Current Definition .
Direct current is an unidirectional continuous current. It is also termed as Galvanic current or plain Galvanic current, as an honor to Luigi Galvani, professor of anatomy who stimulated muscles and nerves of a frog with direct current, in 1786. It is also called constant current, as the current passes continuously in same direction. Direct current is mainly used for iontophoresis, anodal galvanism, cathodal galvanism and for acceleration of healing.
Production of Direct Current .
Direct current can be obtained from dry batteries as well as from household alternating current. From dry batteries: Here dry batteries or cells are connected in series and a variable resistance control output. Voltmeter or Ammeter can be added to this so as to measure the intensity of the output. From alternating current source: Here AC voltage is reduced with the help of a step-down transformer. It is rectified and smoothened to get direct current. Output is controlled via variable resistance and can be measured by voltmeter.
Polarity of Direct Current .
Polarity is generally marked with direct current source by color codes, for example, red for positive and black for negative. It may be tested with two simple experiments such as, salt water experiment and phenolphthalein experiment. In salt water experiment, little salt is added to water in a glass container, then electrodes attached to direct current terminals are inserted. After this, when the intensity is increased, you can see many more bubbles getting evolved at negative electrode or cathode and few bubbles at anode. It is because, water is split into hydrogen and oxygen ions.
Hydrogen being positive ion, gets liberated at cathode and oxygen gets liberated at anode. Considering the water molecule, it is obvious that the number of hydrogen atoms will be double than oxygen and hence, many more bubbles at cathode are evolved as compared to anode. In case of phenolphthalein experiment, add a few drops of phenolphthalein over tissue paper and place the electrodes connected to the direct current source over it, then pass the current for sometime and you will see pink color under negative electrode or cathode.
Physiological Effects of Direct Current .
The various physiological effects of direct current are stimulation of sensory nerves, stimulation of motor nerves, accommodation and chemical production.
Stimulation of Sensory Nerves .
When direct current is applied to the body tissue, it stimulates sensory nerves and produces burning sensation. This sensory irritation is felt under both the electrodes, but if the current flows for sufficient length of time, then more irritation is felt under negative electrode.
Stimulation of Motor Nerves .
Direct current can produce stimulation of motor nerves during its “on” and “off”. Immediately after direct current is switched on, accommodation in motor nerves takes place and hence there is no further response.
Accommodation .
Accommodation is physiological adaptation by nerves. Motor nerves get quickly accommodated to direct current and hence it is not possible to stimulate the innervated as well as denervated muscles by direct current. Analogy of accommodation can be done with our feeling of clothes on the body. We feel that we have put on the clothes after bath, but afterwards we get adapted and do not get the same feel throughout the day. Similarly, if one’s house is by the main road, then he gets used to the night traffic and can have sound sleep as a result of adaptation. Low frequency Currents
Chemical Production .
When direct current is applied to the body tissue it produces chemicals in the form of acid and base. Base or alkali is produced at the cathode and acid at the anode (remember A for acid and A for anode). Medium Frequency Currents
Effect on Body Proteins .
It is believed that the direct current causes coagulation of proteins under positive electrode and liquefaction under negative electrode.
Therapeutic Indications of Direct Current .
Therapeutically direct current is used for medical galvanism, anodal galvanism, cathodal galvanism, surgical galvanism and iontophoresis.
Galvanism.
Use of direct current without any drug for the treatment of various diseases and disorders is known as galvanism.
Medical Galvanism.
Here both the electrodes are of same size . It causes vasomotor stimulation of the skin and increased circulation to the body part where direct current is applied. Medical galvanism is used for acute and chronic inflammatory conditions such as chilblain, Raynaud’s disease, Buerger’s disease, etc.
Anodal Galvanism.
Here anode is an active electrode and cathode, which is larger, acts as an indifferent electrode . Anodal galvanism produces acidic reaction, hardens the tissue and reduces nerve irritability (we will coin a word ‘HARN’ to remember these effects, where H stands for harden, A for acidic reaction and RN for reduce nerve irritability). Anodal galvanism is used for pain relief.
Cathodal Galvanism.
Here cathode is an active electrode and anode is an indifferent electrode . It produces basic reaction or alkaline reaction, softens the tissues and increases nerve irritability. It is used to soften scar tissue.
Surgical Galvanism or Electrolysis .
Surgical galvanism or electrolysis is destruction of superfluous hair (hypertrichosis) by the use of galvanic current. Here cathode, which acts as an active electrode is in the form of needle and anode, is an indifferent electrode. Half milliampere of direct current is applied for one
minute.
Bubbles will come out and then hair can be lifted easily with forceps. Aseptic precautions should be taken during this procedure. Skill is required for surgical galvanism. There are 10 to 20 percent chances of recurrence following this technique.
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