86 ELEMENTS OF LABORATORY WORK 



depend upon the number of turns of wire employed in making 

 the coil, and also upon the diameter of the coil. Other things 

 being the same, the stress varies directly with the number of 

 turns, and inversely as the diameter of the coil. Yery sensi- 

 tive galvanometers are constructed by using a very large 

 number of turns, closely surrounding small magnets delicately 

 suspended by silk or quartz fibres. The deflecting power of the 

 coil is also made more apparent by using several very small 

 magnets pasted at the back of a small mirror, from which a spot 

 of light from a lamp is reflected to a scale. A small movement 

 of the magnets gives a large deflection of the spot of light on 

 the scale. When a steady deflection has been produced in a 

 galvanometer, the stress between the magnet and the earth is 

 just balanced by the stress between the magnet and the coil in 

 an electric condition. 



59. Meaning of ' Conductivity.' If a delicate electrometer 

 is placed in connection with any two points in an electric circuit, 

 it will indicate the existence of a stress. If these two points 

 are altered, so that the length of the conductor between them 

 is increased, the stress will be observed to increase, while it 

 decreases if the length of the conductor diminishes. The 

 greatest value is obtained when the electrometer is connected 

 with the zinc and copper plates themselves (or their equiva- 

 lents). This stress continues so long as the system is unaltered, 

 and so long as the correlative chemical changes in the cell 

 proceed. That is, the same properties are exhibited by every 

 portion of the circuit as are shown by the conductor connect- 

 ing two mutually electrified bodies ; but while the latter con- 

 dition is transient only, the other is maintained so long as 

 certain changes proceed in the cell. 



This stress varies with the nature of the substances form- 

 ing the system, as was observed in the use of the galvano- 

 meter. When a bad conductor, such as bismuth, is inserted, 

 the stress for the same length is much diminished, and may 

 not be perceptible. At the same time, any alteration in the 

 dimensions of the conductor inserted will affect the stress. 

 The thinner the conductor, the smaller the stress, provided 

 other things remain unaltered. Also, addition to the quantity 



