228 Dr. Fleming and Mr. Ashton on a Model which 



shown that for air at atmospheric pressure, a platinum wire 

 at a red heat discharges positive electricity, but not negative. 

 At a higher temperature both positive and negative elec- 

 tricity escape. 



Within certain ranges o£ temperature, therefore, other 

 causes besides unequal velocities of the ions may lead to 

 unequal values of the current with reversal of the electric 

 field. 



Macdonald Physics Building, 

 McGill University, Montreal, 

 March 20, 1901. 



XVJI. On a Model ivhich Imitates tlie Behaviour of Dielectric*. 

 By J. A. Fleming, D.Sc, F.E.S., and A. W. Ashton, 

 B.Sc, 1851 Exhibition Scholar*. 



[Plate V.] 



1. TT has been shown by Profs. Ayrton and Perry, and by 

 X the late Dr. J. Hopkinson and others, that the behaviour 

 of dielectrics with regard to what are termed their "residual 

 charge" effects is analogous to that of wires subjected to 

 mechanical stress. If an iron wire be suspended by one end 

 and a weight applied at the other, it is found that if the appli- 

 cation of the weight has only lasted for a fraction of a second, 

 the wire assumes its original length immediately the weight 

 is taken off, provided the stress has not exceeded a certain 

 limit. If the application of the weight is continued for a 

 considerable time, it is found that the wire does not assume 

 its original length immediately the load is removed ; a small 

 elongation still remains which slowly disappears, the wire 

 very gradually decreasing in length to its original dimensions. 

 in the case of the condenser, if the time of charge is very 

 short, the whole of the charge disappears almost immediately 

 on the condenser-plates being short-circuited. If, however, 

 the charging pressure is kept on for a considerable time,we have 

 first a discharge taking place immediately on short-circuiting 

 the condenser : and this is followed by a slow giving-up of 

 residual charge causing a continually decreasing current 

 through the short-circuiting wire. The stress on the wire in 

 the mechanical experiment is analogous to the electromotive 

 force in the electrical experiment ; the charge of the con- 

 denser corresponds to the elongation of the wire. The rate 

 of variation of the charge, i. e. the charging or discharging 

 current, is therefore analogous to the rate of change of dis- 

 placement of the movable end of the wire, that is the velocity 

 of that end of the wire. 



* Communicated by the Physical Society : read May 31, 1901. 



