44 GENERAL PROPERTIES OF LIVING TISSUES 



order to keep the wire taut, in case changes of tem- 

 perature have caused it to lengthen. (This device is 

 not shown in Fig. 7.) The under surface of the con- 

 tact block is bevelled so that the metal touches the 

 wire only with one edge; the opposite edge is sup- 

 ported by a piece of hard rubber. 



A flexible cable leads from the contact block to the 

 binding post shown in the foreground to the right. 



Fig. 7. The square rheochord ; two-fifths the actual size. 1 



The resistance in the 20 metres of thin Ger- 

 man-silver wire is so great (about 184 ohms) 

 that the internal resistance of the element fur- 

 nishing the electromotive force, together with the 

 resistance of the large copper connecting-wires, 

 practically disappears for such measurements as 

 we shall need to make. As the fall of potential 

 is uniform throughout the 20 metres, the differ- 



i American Journal of Physiology, 1903, viii, p. xli. 



