58 PHYSIOLOGICAL PHYSICS. [Chap. vi. 



this cannot happen, because then the secondary circuit 

 is closed. Not only will unipolar contraction occur 

 when the secondary spiral is open, but it happens also 

 when the spiral is imperfectly closed. Imperfect closure 

 is present when part of the circuit is formed of a bad 

 conductor. Now, the resistance offered by a nerve to 

 the current is great, and it is, therefore, an imperfect 

 conductor. When, therefore, the short-circuiting key 

 is opened, and part of the circuit of the secondary coil 

 is the piece of nerve between the two electrodes, 

 imperfect closure is present, and unipolar contraction 

 is apt to occur and to be mistaken for contraction of 

 the muscle by excitation from the nerve. To prevent 

 this it is recommended to connect the upper of the 

 two electrodes, by means of a good conductor, with the 

 earth. This is effected by leading a short thick wire 

 to the gas-pipe or water-pipe connected with the apart- 

 ment. The free electricity is thus led off and prevented 

 from passing through the nerve and muscle. In any case, 

 where it is desired to make certain that the contraction 

 obtained is due to nervous stimulation, and not to 

 unipolar induction, it is advised, after getting the 

 contraction, to snip through the nerve between the 

 electrodes and the muscle, then cause the cut surfaces 

 of the nerve to make contact with one another, and 

 repeat the experiment. The propagation of the 

 nervous influence is prevented by the section, but the 

 conduction for electricity is still preserved. If, 

 therefore, the first contraction were actually due to 

 nervous stimulus, it will not appear on repeating the 

 experiment ; but if it were due to unipolar action, it 

 will occur just as before. This is called a control 

 experiment, because it tests the accuracy of the result. 

 Instead of cutting the nerve, ligaturing it between the 

 electrodes and the muscle is as effective, since the 

 ligature destroys nervous propagation but preserves 

 electrical conduction. Or the nerve may be kept 



