For diagnostic purposes both galvanic and faradic currents are necessary. The polar method has been applied with great success for determining the condition of nerve and muscle. As has been stated, the pole intended for excitation must be placed over the nerve or muscle to be acted on, while the other rests on some indifferent point—for the upper extremity and trunk, the sternum is convenient. If the operator has no assistant, an interrupting handle of the electrode is useful, or an automatic interrupter may be attached to the pole-board, or the interruptions may be effected by simply applying or removing the electrode.

The normal formulae of nerve and muscle actions have been set forth; in disease these formulae are altered—may be, even entirely reversed. It suffices now to state that when a faradic—an induced— current is applied to a muscle, or to the motor nerve supplying it, contraction of the muscle takes place. This is called faradic excitation, and it is direct when the contraction is caused by application of the electrodes to the muscle itself, and indirect when the motor nerve is acted on. When galvanism is used to produce these effects, the term applied is galvanic excitation, and this is direct or indirect.