chap, vi.] UNIPOLAR INDUCTION. 57 



fashion (Fig. 32). Carry a wire from p to A, another 



from B to the apparatus, and a third from APP to N. 



When the key is opened, the current cannot pass across?. 



from A to B, and is therefore interrupted in APP, and 



when it is closed the current 



passes across the bridge C, and 



the circuit is closed. Thus, 



using the key in this simple way, 



closing means establishing the 



current, while opening it means 



interrupting -the current. 



Unipolar mdetion.-In 

 the stimulation of nerve by 

 induction currents, the key interposed in the circuit 

 ought always to be short -circuited for the preven- 

 tion of what is called unipolar contraction. Sup- 

 pose the circuit of the secondary coil not to be closed, 

 then on the opening and closing of the primary circuit 

 no induction stream can be produced, because of the 

 interruption in the secondary circuit. But it has 

 been shown that in such a case the passing of the 

 current through the primary coil decomposes the 

 neutral electricity of the secondary spiral, and thus 

 free static electricity accumulates at the ends of the 

 secondary wires. This free electricity is of consider- 

 able tension, and will pass off into the earth ; and if 

 it meet a nerve in its course, the nerve will be irri- 

 tated. These conditions are practically fulfilled when 

 the key of the secondary spiral is a simple key, a 

 mercury key, for example. When the key is open, 

 so that no induction stream can be produced, and when 

 a nerve is laid on the electrodes, without proper insu- 

 lation being employed, the nerve is connected with 

 only one pole of the secondary spiral. On the passing 

 off of the free electricity accumulated on the ends of 

 the wires, contraction of the muscle might result. 

 When, however, the secondary coil is short-circuited, 



