258 MEASUREMENT OF CURRENTS. 



It is important to observe that, as comparing the resistances 

 and ', this experiment gives the ratio of the radii of mean action, 

 if the shapes of the channel are simple, so that the terms of cor- 

 rection can be easily calculated. 



877. Formula (20') of (845) gives the equation 



vx 



which would enable us to determine the galvanometric constant by 

 investigating the oscillations for the coil when open and closed, 



provided we know the resistance and the coefficient of self-induction 



M 



of this coil, as well as the ratio of the magnetic moment of the 



rl 



needle to the, horizontal intensity of the field. 



By observing the deflections produced in the galvanometer with 

 a current of known intensity I, we might eliminate from this formula 

 the value of H ; but this method does not seem to possess any 

 practical interest. 



878. REGULATION OF A GALVANOMETER. In galvanometers 

 with astatic needles, or with correction, the sensitiveness may 

 change considerably, either by changes of temperature or by dis- 

 placement of magnets. It would be necessary frequently to repeat 

 the comparison with an absolute instrument ; but this method is 

 not sufficiently expeditious for practical measurements which require 

 no great accuracy. 



The simplest is to use a standard of electromotive force E a 

 Daniell's element, for instance which is connected to the galvano- 

 meter by a suitable resistance R.* If, further, a shunt of multiplying 

 power m is used, and the deflection observed gives a number of 

 divisions on any given scale, the intensity / of the current in the 

 galvanometer is then 



m 



* 20,000 or 30,000 ohms with Thomson's astatic galvanometer (849) and 

 a shunt of a thousandth. 



