1 88 ELECTROMETRY. 



Accordingly the sign of the deflection does not alter when the 

 sign of this difference changes. It must still be remarked that, if we 

 put the two quadrants alternately in connection with two points at a 

 constant difference of potential, the deflection will only be constant 

 provided the metal of the quadrants is the same as that of the 

 needle, for the electromotive force of contact does not change its 

 sign at the same time as the difference of potential of the quadrants. 



If the free pair of quadrants is then connected with the earth, 

 V 1 = 0, and we have 



2C 



In this case the apparatus could be readily used for absolute 



a, 

 measures if the factors could be determined. For this purpose 



2 \^f 



we might revert to the simple form of three cylinders, which we 

 have used to establish the theory of the apparatus. Let us suppose 

 the cylinders vertical, and that the movable cylinder is suspended to 

 the plate of a balance. One of the outer cylinders being insulated, 

 and in electrical connection with the movable cylinder, the other is 

 in connection with the earth. The force P, necessary to keep the 

 beam horizontal, is given by the formula 



but in these conditions, as is readily seen, the sensitiveness of the 

 apparatus would be very small. 



817. In Sir W. Thomson's arrangement the needle is charged to 

 a constant and very high potential (815) ; the two pairs of quadrants 

 are connected with two points whose difference of potential is to be 

 measured. The entire apparatus is in a conducting envelope in 

 connection with the earth, and therefore at zero potential. 



A reversed bell jar is closed by a metal lid, to which all the parts 

 are attached. The needle has, in the direction of its axis, a platinum 

 wire, which dips in concentrated sulphuric acid. This acid forms 



