62O 



A MANUAL OF PHYSIOLOGY 



A compensator is simply a rheocord from which a branch of a 

 current is led off, to balance or ' compensate ' any electrical difference 



in a tissue, like that which gives 

 rise to the current of rest of a 

 muscle, for example (Fig. 209) . 



An electrometer is an instrument 

 for measuring electromotive force 

 that is, differences of electric 

 potential. Lippmann's capillary 

 electrometer is much employed in 

 physiology. A convenient form of 

 it is shown in Fig. 210. A simple 

 form, suitable for students working 

 in a class where a considerable 

 number of copies of the instrument 

 is needed, can be ' conveniently 

 made as follows : A glass tube is 

 drawn out to a capillary at one 



FIG 208. DIAGRAM OF RHEOCORD 

 (AFTER Du BOIS-REYMOND'S MODEL). 



FIG. 209. COMPENSATOR. 



Description of Fig. 208 : I. to VII. are pieces of brass connected with the wires 

 a to / in such a way that by taking out any of the brass plugs i to 5, a greater or 

 less resistance may be interposed between the binding-screws A and B. The 

 two wires a are connected by a slider s, filled with mercury or otherwise making 

 contact between the wires. The current from the battery B divides at A and B, 

 part of it passing through the rheocord, part through N, the nerve, muscle, or 

 other conductor which forms the alternative circuit. When a sufficient resistance 

 R is interposed in the chief circuit to make the total strength of the current 

 independent of changes in the resistance of the rheocord, the strength of the 

 current passing through N will vary inversely as the resistance of the rheocord. 

 When all the plugs are in, and the slider close up to A, there is practically no 

 resistance in the rheocord, and all the current passes across the brass pieces and 

 plugs to B, and thence back to the battery. As s is moved farther away from A, 

 the resistance of the rheocord is increased more and more, and the intensity of 

 the current passing through N becomes greater and greater. The scale S shows 

 the length of wire interposed for any position of s, and this gives a rough measure 

 of the fraction of the current passing through N. When plug i or 2 is taken out, 

 a resistance equal to that of the two wires a is interposed ; plug 3, twice that of a ; 

 plug 4, five times ; plug 5, ten times. 



Description of Fig. 209 : W is a wire stretched alongside a scale S. A battery 

 B is connected to the binding-screws at the ends of the wire. A pair of unpolariz- 

 able electrodes are connected, one with a slider moving on a wire, the other through 

 a galvanometer with one of the terminal binding-screws. In the figure a nerve 

 is shown on the electrodes, one of which is in contact with an uninjured portion, 

 the other with an injured part. The slider is moved until the twig of the com- 

 pensating current just balances the demarcation current of the nerve and the 

 galvanometer shows no deflection. 



