138 Mr Wilson, On a Sensitive Gold-Leaf Electrometer. 



difference of potential between the plate and gold-leaf, in the 

 immediate neighbourhood of which the sensitiveness is a maxi- 

 mum. The less the tilt the greater is this potential and the 

 greater is the maximum sensitiveness, until a certain critical 

 inclination is reached below which there is instability over a 

 certain range of deflections. The range, over which there is insta- 

 bility increases as the instrument is less and less tilted, and when 

 there is no tilt, the plate being then vertical, the leaf is unstable 

 over the greater part of its range. 



The greatest sensitiveness is obtained when the inclination is 

 as little as possible beyond the critical value. 



When the inclination of the apparatus is below the critical 

 value, there is a certain range of potential differences between the 

 plate and the gold-leaf within which there are, for a given potential 

 difference, two positions of stable equilibrium for the leaf. The 

 behaviour of the instrument under these conditions will be under- 

 stood more readily with the aid of the diagrams which follow 

 (fig. 4). For these, it must be understood, only a qualitative 



Fig. 4. 



correctness can be claimed ; they show the general nature of the 

 curves connecting deflection with difference of potential. The 

 portion of the curve with which we are specially concerned is 

 S-shaped. In the central dotted portion of the S, where the 

 deflection increases as the voltage diminishes, we get instability, 

 and the curve here cannot be followed experimentally. The rest 

 of the curve shown by full lines can be experimentally realized. If 

 we start with a small difference of potential and gradually in- 

 crease it, the deflection increases more and more rapidly as the 

 point B is approached. At this point the gold-leaf is found 

 suddenly to jump to the position D, and if the potential difference 



