PRELIMINARY DATA 



701 



The string galvanometer of Einthoven has peculiar merits for certain 

 physiological purposes. It consists of a silvered quartz- or glass-fibre 

 stretched in a very strong magnetic field. When traversed by a current 



the fibre is deflected, and by means 

 of a beam of light the deflection is 

 greatly magnified (Fig. 224). 



A rheocord is an instrument by 

 means of which a current may be 

 divided, and a definite portion of it 

 sent through a tissue (Fig. 225). 



A compensator is simply a rheo- 

 cord from which a branch of a current 

 is led off , to balance or ' compen- 

 sate ' any electrical difference in a 

 tissue, like that which gives rise to 

 the current of rest of a muscle, for 

 example (Fig. 226). 



An electrometer is an instrument 

 for measuring electromotive force 



Fig. 225. Diagram of Rheocord (after 

 Du Bois-Reymond's Model). 



Fig. 226. Compensator. 



Description of Fig. 225 : 1 to VII are pieces of brass connected with the wires a to f 

 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 father away from A, the resistance of the rheocoid 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. 226: 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 unpolarizable 

 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 compensating 

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

 shows no deflection. 



