OBSERVATION OF DEFLECTIONS. 261 



The intensity / of the current in the galvanometer being 



x 



we get from it the equation 



which gives the coefficient R a , by the known resistances g lt b, b', c, 

 and by the differences a' - #, V b. 



A more direct method is to observe the current in the first con- 

 dition, and then cut the conductor c and replace the resistance b 

 by a resistance t> 19 such that the deflection does not vary. We 

 have then as a condition 



and there are only four resistances to measure. 



880. OBSERVATION OF DEFLECTIONS. With galvanometers the 

 damping of which is rapid, equilibrium is established after so short 

 a time that the deflection for the permanent current is observed 

 without difficulty. In other cases, an auxiliary damping magnet or 

 current (842) is used ; but we may also employ the current itself 

 by one of the following methods, which are due to Gauss,* and 

 which are particularly suitable for observing magnetic systems the 

 moment of inertia of which is considerable. 



The passage of the current suddenly displaces the needle through 

 an angle 8 from its position of equilibrium. When the motion is 

 that of a pendulum, the arc of the first throw relative to the 



permanent current is 28 ; but if this current is broken when the arc 



8 

 traversed is - , which corresponds to a third of the time of oscil- 



* GAUSS. Resultate aus den Beobachtungen des Magn. Vereins. 1839. 

 CEnvres, Vol. V., p. 395. 



