The chief use of electricity in diagnosis is dependent upon its power of evolving muscular contraction. We have seen--as I demonstrated to you upon my left forearm--that both muscles and nerves, when in a normal condition, respond to the stimulus of electricity. In disease this response, or irritability, may remain unaltered, or it may be increased, decreased, or abolished; and our first step in electro-diagnosis should therefore be to ascertain the exact condition of electro-contractility. As in practically almost all our cases we have to do with altered reaction existing only in one nerve or muscle, or in one of two symmetrical groups of nerves or muscles, we possess a ready means of testing irritability by ascertaining its relative condition upon opposite sides of the body, as, for example, in an ordinary case of hemiplegia. [SN: Test of Farado-Irritability.] In examining such a case electrically--and the principles of procedure are the same in all cases--it is convenient to commence our investigation with the Faradaic current, and to ascertain the _lowest_ power which will call into action any one of the muscles of the healthy side, and then to apply this same strength of current _to identical points of the corresponding muscle on the diseased side_, noting whether it causes contraction. If so, we decrease the power of the current, when if contraction still occurs there is increased irritability, or _vice versá_, as the case may be. Having ascertained the condition of the muscle we proceed similarly to test its motor nerve, for we shall see hereafter that in certain diseased conditions muscle and nerve irritability are affected unequally. One conductor being held to an indifferent part of the body we apply a fine-pointed conductor to the most superficial point in the course of the nerve, in the way that, as you will recollect, I electrized my left median nerve. [SN: Test of Voltao-Irritability.] We next submit the muscle and nerve to similar examination with the Voltaic current. Holding both conductors immovable, we interrupt the current for a moment by moving the “key” of our battery, or if it is not provided with a key, while one conductor is stationary, we break the current by lifting and reapplying the other. It is essential that on both sides there should be exact similarity in the application, and that the electrodes should be placed on identical points of the muscle, and this is especially important with the Voltaic current, for healthy muscle responds to it more readily when it flows down the limb, that is, with the positive pole nearest to the spine, and the negative farther from it than when it flows up, and consequently a reversal of the poles will influence the result. [SN: Rule for strength of Current.] In testing a case in which there is equal disease upon both sides--as in some cases of paraplegia--we must be guided by a knowledge of the strength of current usually required to induce contraction.