PRELIMINARY DATA 



699 



other, and with a galvanometer, G, and a battery, F, no current will 

 flow through the galvanometer when TTS= ^F)' 



In making the measurement, a resistance box, containing a large 

 number of coils of wire of different resistances, is used (Fig. 221). The 

 resistances corresponding to AB and AD may be made equal, or may 

 stand to each other in a ratio of i : 10, i : 100, etc. Then, the un- 

 known resistance being CD, BC is adjusted by taking plugs out of the box 



Fig. 220. Wheat- 

 stone's Bridge. 



Fig. 221. Diagram of Resistance Box. 



till, on closing the current, there is either no deflection, or the deflection 

 is as small as it is possible to make it with the given arrangement. 



Galvanometers. A galvanometer is an instrument used to detect a 

 current, to determine its direction, and to measure its intensity. Since, 

 by Ohm's law, electromotive force, resistance, and current strength 

 are connected together, any one of them may be measured by the gal- 

 vanometer. A galvanometer of the kind ordinarily used in physiology- 

 consists essentially of a small magnet suspended in the axis of a coil 



Fig. 222. Scheme of Wiedetnann's Galvanometer (with Telescope Reading). T, 

 telescope; S, scale; M, mirror; w, ring magnet suspended between the two gal- 

 vanometer coils G, the distance of which from m can be varied; F, fibre suspend- 

 ing mirror and magnet. 



of wire, and free to rotate under the influence of a current passing 

 through the coil. The most sensitive instruments possess a small 

 mirror, to which the magnet is rigidly attached. A ray of light is 

 allowed to fall on the mirror, from which it is reflected on to a scale; 

 and the rotation of the mirror is magnified and measured by the ex- 

 cursion of the spot of light on the scale. In the Thomson galvanometers 

 the magnet is very light e.g., a strip or two of magnetized watch 

 spring. The magnet is ' damped ' that is, its tendency, when once 

 displaced, to go on oscillating about its new position of equilibrium is 



