IRRITABILITY. 



447 



duced in the secondary coil, which is manifested by a contraction 

 of the muscle. The effect upon the muscle is brief, and it returns 

 to its former condition, and so long as the current is flowing no 

 further change takes place in it; but the moment the primary 

 current is broken the muscle again contracts, because of the pro- 

 duction of another induced current. It will be recalled that with 

 the direct battery current the closing of the circuit, or the make, 

 produced the greater effect upon the muscle ; in the induced 

 current it is the break which produces the more powerful shock. 



Du Bois-Reymond* s Indudorium (Fig. 250). This induction 

 apparatus is the one most commonly used in physiologic labora- 

 tories. 



To render the make and break shocks of the secondary coil 



FIGS. 252, 253. Pohl's mercury commutator. 



more equal, Helmholtz connected one pole of the battery with v 

 (Fig. 250), and the other with A, and A and v with a short and 

 thick wire. On account of the wire between v and A, the primary 

 current is never opened, but passes through the primary coil, and 

 when the vibrating spring and v come into contact, the current is 

 short-circuited. 



Non-polarizable Electrodes. When a muscle or other tissue 

 is placed upon metal electrodes through which a current is passing, 

 the electrodes become polarized as a result of the decomposition 

 taking place in the tissue, and consequently currents are set up 

 which materially interfere with a proper interpretation of the 

 effects of the current from the battery or from the induced cur- 

 rent, as the case may be. To avoid this, special forms of electrodes 

 have been devised which are known as unpolarizable or non- 

 polarizable electrodes. Fig. 251 shows such electrodes. Each one 



