726 THE PHYSIOLOGY OF 711 L CONTRACTILE TISSUES 

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



A T$ 'R'"* 

 flow through the galvanometer \\lien YTS = QFJ- 



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

 number of coils of wire of different resistances, is used. 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 unknown resistance being CD, BC 

 is adjusted by taking plugs out of the 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 galvanometer. A galvanometer of 

 Fig. 231. Wheat- the kind ordinarily used in physiology consists essen- 

 stone s Bridge, tially of a small magnet suspended in the axis of a 

 coil 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 oi the mir- 

 ror is magnified and 

 measured by the ex- 

 cursion of the spot 

 of light on the scale. 

 The method of read- 

 ing by a telescope 

 can be applied to any 

 mirror galvanometer, B 

 and is often extreme- 

 ly convenient in 

 physiological work. 

 Sometimes a small 

 scale is fastened on 

 the mirror itself, and 

 o'bserved directly 

 through a low-power 

 microscope. 



Fig. 232. Diagram of String Galvanometer. The string or fibre CC is stretched be- 

 tween the poles of a powerful electromagnet. When a current passes down the 

 string it is deflected in the direction of the larg arrow a i.e., at right angles to 

 the magnetic field NS. When the current is reversed, the string moves in the 

 opposite direction. The movements of the string can be observed by a micro- 

 scope, A (objective E), passing through a hole bored through the centre of the 

 magnet poles. For obtaining records a source of light is placed at B and con- 

 centrated on the fibre by a condenser, F, and the movements of the shadow are 

 recorded by photography. 



In the d'Arsonval galvanometer the current passes through a small 

 coil of fine wire suspended in the field of a strong magnet. When 

 the current passes the coil is deflected, carrying with it a small mirror 

 attached to the suspending filament. A great advantage of this galvano- 

 meter in many situations is that it is unaffected by neighbouring currents. 



