UOOTABILITY* 



u: 



duood in tho secondary coil, \vhioh is manifested by a contraction 

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

 to its former condition, and so long as the onrront 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 lottery current the closing of the circuit, or the ttu?&, 

 produced tho greater effect upon the muscle; in the induced 

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



Iht tfoiV/frywowfs Indudorium (Fig. 250), This induction 

 apparatus is the one most commonly used in physiologic labora- 

 tories. 



To render the make ami break shocks of the secondary coil 



I 



*. 353, m Pohrs mercury commutator. 



more equal, Helmholtx connected one |x>lo of the Ivitterv with r 

 (Fig. 2oi>), and the other with A, and A and r with a short and 

 thick wire. On account of the wire between r and .1. the primary 

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

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

 short -circuited. 



^fatHMtorimftfc Ettctmlcs. When a muscle or other tissue 

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

 the electrodes hoeome polamed as a result of the decomposition 

 taking place in the tissue, and consequent!? 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 ease may bo. To avoid this, special forms of electrodes 

 have boon devised which are known as HnpoliirizaMt or MOH- 

 polarisabl* dectrod(& Fig. -M shows such electrodes. Kaeh one 



