152 



THE SIGNS OF LIFE 



It is advisable to fit up a galvanometer once and for all in one 

 particular place. The best position for the instrument, and one 

 in which it will always be ready to work, is either a recess or small 

 cupboard built in the thickness of a perfectly firm wall. Failing 

 this, a firm bracket or shelf, not connected with the floor, is sufficient. 



Whether the instrument is being used for purposes of demonstra- 

 tion or for actual measurement, the objective method must be used. 

 This consists in the projection of a beam of light either on to a trans- 

 parent and graduated scale or upon a photographic plate. The most 



FlG. 56. Normal circuit described in the text. (The secondary coil is 

 figured as if for direct excitation of a given object, IV. Obviously, if we 

 have to apply indirect excitation, the wires are removed from the keyboard, 

 and the plug hole is filled to complete the keyboard circuit.) 



convenient light for either purpose is the image of the filament of an 

 incandescent lamp. When photographic records are required, it is 

 very desirable, indeed almost indispensable, to work with two galvano- 

 meters in series, one for purposes of observation, the other as record- 

 ing instrument. 



Calibration of the galvanometer, and choice of a convenient scale. 



-The deviations of the same galvanometer have not always a 



constant value, seeing that the resistance of the circuit varies 



according to the resistance of the object under examination and 



of the unpolarisable electrodes. 



The quickest way to calibrate a galvanometer is to observe or 

 photograph the deflection of the spot when a current of known 



