202 Mr. M. Donovan on the supposed Identity of the Agent 



in short, we are totally unacquainted with its form of existence. 

 We can only judge of what, for want of a better phrase, we call 

 its quantity, by the degree of its effects upon matter of determi- 

 nate dimensions. The degree of effect is therefore the exponent 

 of the quantity. But this degree is another name for intensity 

 of effect : hence the quantity of electricity is only recognised by 

 its intensity ; the ideas are inseparable ; and intensity is the only 

 significant condition of which the external senses take cognizance. 

 In fine, intensity is the measure of quantity; and of this a 

 beautiful experimental proof is given by Pouillet, which it is well 

 worth while here to describe. He proposes the following ques- 

 tion, and gives its resolution : — 



"When the intensity of a current increases, does the quantity 

 of electricity which is in circulation, and which constitutes that 

 current, increase in the same ratio ? To resolve this question, it 

 must be admitted as evident that the quantity of electricity 

 which passes in a circuit of constant intensity is proportional to 

 the time, that is to say, that in 2" there passes twice as much as 

 in 1", &c. It suffices, then, to examine if, in reducing the time 

 during which a current passes to one-balf, we equally reduce to 

 one-half its action on the needle ; for if it be so, we may truly 

 affirm that the quantity of electricity is proportional to the elec- 

 tro-magnetic effect of the current, or to its intensity." 



He then describes some beautifully contrived experiments 

 calculated to ascertain this point. A brass toothed wheel, having 

 the spaces between the teeth filled up with wood, was constructed ; 

 the brass teeth and those of wood were in this instance of equal 

 size, although other wheels were also prepared in which the brass 

 and wooden teeth bore various ratios to each other. The edge 

 of the wheel was united like the periphery of a disc, and thus 

 alternately presented conducting and non-conducting surfaces. 

 This wheel with its metallic axis was contrived to revolve with 

 any required degree of rapidity ; one pole of a voltaic pile com- 

 municated with the axis, and the other with a wire, which, after 

 passing over the needle of a compass, terminated in a tongue, 

 and this pressed against the edge of the wheel. By giving the 

 Avhecl a very slow motion, the needle oscillated backward or for- 

 ward as the tongue came in contact with the wood or the brass, 

 the current being interrupted or conducted accordingly. The 

 interruptions were obliterated and a permanent deflection pro- 

 duced when a greater velocity was given to the wheel ; and this 

 deflection was not increased by a much more rapid revolution. 

 In an experiment made with the. wheel, in which the peripheral 

 surfaces of the brass and wooden teeth were equal, the deflection 

 of the needle while the wheel was at rest was 60° ; a slow motion 

 made it oscdlate ; five turns per second caused a deflection of 



