286 MEASUREMENT OF CURRENTS. 



directions, and following each other with sufficient rapidity, the 

 mean intensity of the current is equal to the algebraical sum of 

 the quantities of electricity which pass in each unit of time. It 

 is zero, as a particular case, if the successive discharges are equal 

 and opposite in direction. This would be the case of a sinusoidal 

 current, or more generally of a periodical current of any given 

 form, the intensity of which changes in sign, while retaining the 

 same value, at the end of a half period. 



It would likewise be so with an induction coil, such as that of 

 Ruhmkorff, in a closed circuit ; for the quantities of electricity 

 which correspond to the two kinds of induced currents are 

 equal (541). If, further, the circuit contains a constant electro- 

 motive force, the deflection is the same as if this electromotive 

 force alone existed.* 



It may, in passing, be observed that this experiment is an 

 indirect verification of the theory. It shows that the resistance 

 of the circuit is not a function of the intensity, for at each instant 

 the current only depends on the algebraical sum of the electro- 

 motive forces. 



901. BILATERAL DEFLECTION. It happens, especially with 

 astatic galvanometers, that for a current whose mean intensity is 

 zero, the needle is in unstable equilibrium at zero, and that, once 

 deflected, it diverges to 90. This fact, which was observed by 

 Poggendorff,t and called by him bilateral deflection, is due to the 

 temporary magnetism developed in the needle by the current. It 

 has been investigated and examined in a .complete manner by 

 Professor Chrystal.J 



When the external field is weak and the alternate current 

 sufficiently strong, the needle leaves its position of equilibrium 

 and deflects through 90 in one direction or the other. This is 

 the phenomenon first observed by Poggendorff, and which Pro- 

 fessor Chrystal calls bilateral deflection. 



If the intensity of the current is diminished, or the intensity 

 of the magnetic field is increased, two cases may present them- 

 selves, according to the initial position of the needle : i. If it is 

 exactly in the plane of symmetry of the coil it is stationary, not- 

 withstanding the passage of the current ; 2. If its initial direction 



* SCHUSTER. Phil. Mag. [4], Vol. XLVIII., p. 254. 1874. JAMIN. Comptes 

 rendus, Vol. xciv., p. 1616. 1882. 



t POGGENDORFF. Fogg. Ann., Band XLV., p. 353. 1838. 

 CHRYSTAL. Phil. Mag. [5], Vol. n., p. 401. 1876. 



