76 



ELEMENTS OF LABORATORY WORK 



when there is an electric stress between two bodies ; for then 

 the vane or indicator moves, together with the spot of light on 

 the scale. But we may also determine by its use a much more 

 important matter, viz. which of two electric stresses is the 

 greater. If one pair of quadrants is introduced into an electric 

 system, the indicator will move towards or away from that 

 pair, according to circumstances. If the other pair of quadrants 

 is introduced into another electric system, so as to occupy a 

 corresponding position in it, there will be a stress between 

 this pair and the indicator which will oppose the first stress. 

 The deflection of the indicator will show the relation between 

 these stresses. 



In the attracted-disc electrometer, the electric stress is 

 compared with that of gravity a stress which may be easily 

 defined and measured, since it varies directly with the quantity 

 of matter used. The electric stress between two opposed 

 discs is accurately balanced, by a lever arrangement, with the 

 stress due to a certain quantity of matter and the earth. 



The stress existing between parts of a system similarly 

 electrified may also be exhibited by the instrument shown 

 in the diagram. A thick copper or 

 brass wire circle or ellipse has two 

 plates, A and B, attached in such a 

 position that similar plates, c and D, 

 at the end of a thin pointer may 

 face them. This pointer is carefully 

 balanced on the steel point E, and the 

 whole is now supported on an ebonite 

 pillar F. We obtain a system in com- 

 plete electric connection, and able to 

 exhibit stress between its parts with- 

 out losing that connection. It should 

 be covered just as the gold-leaf elec- 

 troscope by a glass coated inside with 

 strips of tinfoil. A plate G is at- 

 tached so that the system may be electrified with convenience, 

 whereupon, the pointer is repelled to a distance varying with 

 the magnitude of the electric stress. Observations made with 



Fig. 25. 



