145 147] ELECTROMETERS. 281 



between A and B the field is uniform. A being then discon- 

 nected from C, its charge may be used by itself. The outer part 

 G is called the guard-ring, and its function is to render the field 

 uniform all over the working conductor A. In order that there 

 may be no charge on the back of A, the guard-ring C is made 

 part of a closed conducting box D, which has no charge in its 

 inner surface, hence none on the back of C and A. The principle 

 of the guard-ring is due to Lord Kelvin*. It may equally well be 

 applied to the cylindrical condenser, by separating a portion of the 

 outer cylinder from the ends, which are connected with an envelop- 

 ing annular box. 



147. Absolute Electrometers. The potentials of the 

 plate of a plane condenser being Fj and F 2 , the energy is 



The force tending to increase T is 



The negative sign shows the force to be an attraction. If the 

 working plate be hung from a balance, and counterbalanced by the 

 weight of a mass M, 



V V - 



r i * 2 



We thus have an electrometer, or instrument for the purpose of 

 measuring differences of potential. Lord Kelvin's f original instru- 

 ment has the plate B carried by a micrometer screw, so that r can 

 be varied, while A is hung from a system of springs, whose 

 tension, replacing Mg, is constant. In this case Fj F 2 is directly 

 proportional to r. In the balance form, used by Rowland and 

 others, T is constant, and Fj - F 2 is proportional to JM. We 

 have in this case a practical difficulty, in that if the upper plate 



* Electrometers and Electrostatic Instruments. B. A, Report, 1855. Papers 

 on Electrostatics and Magnetism, p. 263. 

 t loc. cit. 358. 



