296 MEASUREMENT OF CURRENTS. 



Von Helmholtz used a coil of thick copper wire, which was made 

 to act on a magnetised needle intended to measure the discharge 

 for a transient current. A key closes the circuit and opens it after 

 a time 0; we have then (895) 



The experiment is then begun again with this difference, that 

 instead of opening the circuit after a time 0, an equal resistance 

 is substituted for the battery. The new swing corresponds to the 

 quantity of electricity w = I 0. The ratio of the two swings gives 

 the ratio of the mean current during the time to its final value. 

 From this is* easily deduced the intensity at each instant. 



910. In order to make or break circuits at known intervals, 

 we may use bodies which have a motion of translation, as the fall 

 of a weight, or oscillating or rotating apparatus. One of the most 

 convenient is a very heavy pendulum, which is allowed to fall 

 from a given height, and which is allowed to act on contact studs 

 at the time of its greatest velocity. 



In the case of a current on closing, for instance, the pendulum 

 first strikes against a lever, which closes the circuit and marks the 

 origin of the phenomenon, and then against another one which 

 opens the circuit. This second lever may be displaced parallel 

 to itself by means of a micrometric screw. The interval which 

 separates the two contacts is deduced from the time of oscillation 

 and the distance of the levers. 



911. This arrangement was utilised by Von Helmholtz,* and 

 after him by Schiller,! in investigating the electrical oscillations 

 produced in an open circuit when the current in an adjacent one 

 is broken. Let A be the inducing coil, A' the induced coil, the 

 ends of the wire in which are connected with the armatures of a 

 condenser. When the inducing current is opened, an electromotive 

 force is developed in the circuit A', which conveys opposite elec- 

 tricities to the two armatures. The capacity of the wire itself is 

 the charge of each of its halves for unit difference of potential 

 at the fwo ends. If these communicate with a condenser, the 



* HELMHOLTZ. VerhandL der Naturhist. Median. Vereins zu Heidelberg, 

 Vol. v., p. 27. 1869. Wissenschaft. Abhandl., Vol. I., p. 531. 

 t SCHILLER. Pogg. Ann., Vol. CLII., p. 535. 1874. 



