﻿Galvanometer-Needle for Transient Currents. 203 



As a knowledge o£ these effects is a great help in under- 

 standing the action of transient currents, whether direct or 

 oscillatory, on a galvanometer-needle or coil, we shall give a 

 brief resume of the main phenomena and indicate the theory. 



Effect of Alternating Currents on a Galvanometer- Needle 

 or Coil. 



Let us suppose that we have an ordinary Kelvin mirror- 

 galvanometer, and that we connect it with the alternating- 

 current mains through a high resistance. Let us first 

 suppose that the mirror, and consequently the needle, is at 

 right angles to the axis of the galvanometer coil, and that it 

 is in stable equilibrium in this position before we close the 

 switch. If the effective value of the alternating current be 

 less than a certain critical value, the needle will still be in 

 stable equilibrium after closing the switch, but the time of 

 swing will be longer. As the current approaches the critical 

 value the time of swing gets longer and longer, and when it 

 equals the critical value the equilibrium is neutral. For 

 greater values of the alternating current the spot of light 

 moves off the scale to one side or the other. This is what 

 Professor Chrystnl calls bilateral galvanometer deflexion. 

 If the needle be in stable equilibrium, the effect of the 

 alternating field is to make it more sensitive to magnetic 

 impulses. 



If we now twist the fibre or move the controlling magnet 

 so that the spot of light is no longer in the centre of the 

 scale initially, then, when we increase the alternating current, 

 the spot of light moves steadily away from the centre of the 

 scale to a new position of equilibrium. This is called uni- 

 lateral deflexion. 



Similar effects are produced by alternating currents in the 

 coil of a d'Arsonval galvanometer. In this case, when the 

 spot of light is in the centre of the scale the coil is in stable 

 equilibrium. The unilateral deflexion, however, is generally 

 towards the centre of the scale. If the moving coil be 

 enclosed in a damping metallic cylinder, a small unilateral 

 deflexion away from the centre of the scale can sometimes be 

 observed. 



To explain these effects let us first consider the case of a 

 Kelvin mirror-galvanometer. Let MP be the moment of 

 inertia of the mirror and needle about an axis through the 

 suspending fibre, let be the initial angular deflexion of 

 the mirror and 6 its deflexion at the time t. The equation 

 determining the motion is 



MP0 + 260 + //,B sin(6>-^ ) = ^cos^ + 7rsin^cos6>. (1) 



