ELECTEOMETEES. 237 



The mirror is a small concave mirror of silvered glass. 

 It is about the size of a threepenny-piece. It is extremely 

 light, weighing only about ,a third of a grain. To make 

 these little glass concave mirrors (which are also used for 

 reflecting galvanometers, where extreme lightness is even 

 more essential than in the present case) was a matter of 

 considerable difficulty. The following plan is now adopted. 

 A large number of little circles of the finest micro- 

 scope glass are silvered. When they are to be used for 

 reflecting galvanometers, four minute magnets are cemented 

 to the back. The mirrors are then tested, to see that each 

 gives a good image of a lamp reflected from it ; and, out of 

 perhaps fifty tried, ten or fifteen may be found satisfactory. 

 This plan of selection by trial gives mirrors that afford a 

 perfect image, and which are lighter than any that can be 

 obtained by the plan of grinding at first adopted. The 

 concavity of the mirrors is such that the rays of a lamp 

 placed about one metre (39-J- inches) from the mirror, and 

 reflected from it, are brought to a focus on a screen at the 

 same distance from the mirror. 



In the use of the electrometer a lamp is placed in front 

 of the mirror, and its rays, passing through a narrow vertical 

 slit, fall on the mirror and are reflected from it. The reflected 

 rays are brought to a focus on a long horizontal screen at 

 right angles to the line from the slit to the mirror, and just 

 above the slit. The screen, which has marked on it a 

 finely-divided scale, is one metre distant from the mirror. 



Now you will see that the position of the image of the 

 slit on the scale depends upon the angle at which the rays 

 proceeding from the slit to the mirror fall upon the mirror ; 

 and as the mirror turns with the needle, the position of the 

 image on the scale depends upon the position of the needle. 

 When the needle is at its position of zero deflection, the 

 image of the slit is seen at a point on the scale just above 

 the slit. When the needle is deflected to one side or other, 

 the image on the scale is also deflected, and by a well-known 

 optical principle the angle of deflection of the reflected rays 

 is twice the angle of deflection of the mirror, that is of 

 the needle. 



We are now prepared to understand the principle on which 

 the use of the quadrant electrometer depends. We have 

 in this case three bodies, two of them fixed (the two pairs 



