RECENT PROGRESS IN PHYSICS. 



451 



ignition in the wire, visible by daylight, the thermometer being 

 observed each time. The same was repeated with different number 

 of jars. Such a series of observations gave the folloAving charges 

 required to produce ignition with the corresponding depressions of 

 the thermometer. 



produce the the same effect if -^ =: —j 



s s 



To bring the thin wire to a visible red heat, the quantities of elec- 

 tricity 12 in 5 jars, 11 in 4_, 10 in 3, and 8 in 2, were necessary. 

 Dividing the square of the quantities of electricity by the correspond- 

 ing number of jars, we get the following quotients : 



144.5 = 28.5, 121.4 == 30.2, 100.3 =z 33.3, 64.2 = 32. 



These quotients are very nearly equal, and from this we may infer 

 that, if a quantity of electricity gins jars make a wire red hot, under 

 circumstances equal in other respects, the quantity g' in s' jars will 



In the above experiments 



the mean of the quotients is 31 ; hence, for 



jars 2, 3, 4, 5, 6, 7 



the quantities of E 7.9, 9.6, 11, 12.4, 13.6, 14.7 



will be found as those required to produce incandescence in the above 

 mentioned wire. 



We have seen above that the heat produced by a discharge through 

 the electrical thermometer, under otherwise like circumstances, 



remains the same so long as the quotient -^ , or what is the same, the 



s 



product of the electrical quantity q multiplied by the density -^ does 



not change. But since the same value of ^ is always necessary for 



s 



the ignition of the thin wire, it was to be supposed that the discharges 

 which effect the ignition of the thin wire also produced in the ther- 

 mometer like temperatures, which, in fact, is very nearly the case in 

 the above series of experiments. 



For the sake of brevity we shall measure the current by its heating 

 power, and always denote the quantity of heat produced in a wire, kept 

 constantly in the conducting circuit, by the term force of the discharge. 



This force is constant so long as the value of , ^ - does not vary, 



other things being equal 



2. Ignition of the ivire in proportion to its length, — When a dis- 

 charge current produces incandescence in a thin wire, a prolongation 

 of the wire retards the current so that the glow no longer appears. 



