1 66 ELECTROMETRY. 



PART II. 

 ELECTRICAL MEASUREMENTS. 



CHAPTER I. 

 ELECTROMETRY. 



796. GENERAL CHARACTERISTICS OF ELECTROMETERS. An 

 electrometer is an instrument intended to measure either a quantity 

 of electricity, or the difference of potential which exists between 

 two conductors in equilibrium, or between two points of a con- 

 ductor not in equilibrium. 



The simplest electrometer, which is also the oldest and the 

 most in use, consists of an insulated conductor to which are 

 suspended two light bodies for instance, a double pendulum or 

 two gold leaves. It has usually in addition two fixed conductors, 

 called the terminals, placed symmetrically in respect of the gold 

 leaves and communicating with the earth. 



797. GOLD LEAF ELECTROSCOPE. The gold leaves are ordi- 

 narily surrounded by a glass bell jar, which serves both as insulating 

 support and as a protection against currents of air. 



This bell jar contains drying substances, ordinarily lumps of 

 burnt lime or of calcium chloride. It is sometimes surrounded 

 by a second shade provided with a hole through which the rod 

 passes, and which itself contains hygroscopic substances to protect 

 the outside of the bell jar against moisture. The object of all 

 these precautions is to insulate the gold leaves. 



The use of a glass shade may present inconveniences in conse- 

 quence of traces of electricity met with on the sides. This electrifi- 

 cation is produced with the greatest facility on very dry glass. 



It is better to use a conducting shade, such as one of wire gauze, 

 connected with the terminals and with the ground, the stem passing 

 through an aperture, and being supported, for instance, by a glass rod 



