88 PHYSIOLOGICAL PHYSICS. [Chap. vm. 



of investigation, it is found that the occurrence 

 of a contraction, and the amount of the contraction, 

 whether feeble or strong, depend ( 1 ) on the strength of 

 the current, and (2) on the direction of the current. 

 The rheocord gives the simplest means of graduating 

 the strength of the current, and the commutator the 

 means of reversing it. The diagram (Fig. 47) shows 

 how the arrangement ought to be made. R repre- 

 sents the rheocord, c a commutator, with cross, and 

 E the galvanic elements, which may be 3, 4, or other 

 number, of the small Grove elements, all arranged 

 precisely as shown in the diagram. The muscle-nerve 



Fig. 47. Arrangements for Studying the Law of Contraction. 



preparation M is arranged in the telegraph, and the 

 nerve is laid over the platinum electrodes, which 

 are connected with the wires coming from the 

 rheocord. Now by altering the number of Grove's 

 elements, and especially by altering the position of 

 the rheocord slider, and by means of the plugs, as 

 explained (page 81), no current may be sent to the 

 nerve, or a current may be sent whose strength may 

 be graduated to any desired extent. To open and 

 close the current, a simple mercury key is interposed ; 

 or, better still, in order to make or break the current 

 always with the same rapidity, Pflueger's fall hammer 

 (page 69) may be used as a key. Make the experi- 

 ments in the following way. 



By means of the rheocord send only a very weak 

 current to the nerve ; arrange by means of the com- 

 mutator that the current shall pass down the nerve, 

 close the key, and note the result ; then open the key, 



