252 ELEMENTARY LESSONS ON [CHAP. iv. 



of an electric "effluvium" or fluid ; it is now known to be 

 due to the presence of ozone, a modified form of oxygen 

 gas, which differs from oxygen in being denser, more 

 active chemically, and in having a characteristic smell. 

 The discharge of the Holtz- machine and that of the 

 induction coil are particularly favourable to the pro- 

 duction of this substance. 



299. Dissipation of Charge. However well in- 

 sulated a charged conductor may be, and however dry 

 the surrounding air, it nevertheless slowly loses its 

 charge, and in a few days will be found to be completely 

 discharged. The rate of loss of charge is, however, not 

 uniform. It is approximately proportional to the dif- 

 ference of potential between the body and the earth. 

 Hence the rate of loss is greater at first than afterwards, 

 and is greater for highly charged bodies than for those 

 feebly charged. The law of dissipation of charge 

 therefore resembles Newton's law of cooling, according 

 to which the rate of cooling of a hot body is propor- 

 tional to the difference of temperature between it and 

 the surrounding objects. If the potential of the body 

 be measured at equal intervals of time it will be found 

 to have diminished in a decreasing geometric series ; or 

 the logarithms of the potentials at equal intervals of time 

 will differ by equal amounts. 



This may be represented by the following equation : 

 V V e ~ pt 



V t V o e 9 



where V represents the original potential and V t the potential 

 after an interval /. Here e stands for the number 2*71828 . . . 

 (the base of the natural logarithms), and p stands for the " co- 

 efficient of leakage," which depends upon the temperature, 

 pressure, and humidity of the air. 



The rate of loss is, however, greater at negatively 

 electrified surfaces than at positive. 



3OO. Positive and Negative Electrification. 

 The student will not have failed to notice throughout 



