146 Intelligence and Miscellaneous Articles. 



Seeking to make some comparisons of this sort, I have employed 

 the dynamoelectrical machine of the G-eneva University, which was 

 set to work by first passing the current through a Serrin lamp, or 

 auy other equivalent resistance, until the machine had acquired its 

 normal velocity. Then, by means of a commutator, the current 

 was directed through a platinum wire, while the lamp was excluded 

 from the circuit. The wire grew rapidly hot, and soon melted. 



The dynamoelectric machine is driven by an hydraulic motor of 

 nominal four-horse power. All this force is far from being em- 

 ployed ; but let us suppose that it is. A four-horse power corre- 

 sponds to a work of 18000 kilogram metres per minute, equivalent 

 to 42*3 calories. Suppose, further, that the dynamoelectric ma- 

 chine is perfect, and that all the motive work is converted into 

 electric current ; neglect the heat evolved in the machine itself and 

 in the conductors that convey the current to the platinum wire, 

 upon which we will suppose that the whole effect is concentrated. 

 That wire will therefore receive, as a maximum, 42-3 calories (a 

 number which in reality is certainly four times too much). 



The diameter of the platinum wire was 0*31 millim. ; therefore 

 its cylindrical surface was 1 square millim. per millim. of current. 

 The length of the wire, in three experiments, was 385 millims. It 

 was melted in a few seconds, and broke at several points. On after- 

 wards examining its fragments, traces of liquefaction were every- 

 where recognized. We may therefore conclude that the whole of it 

 was raised to the temperature of fusion of platinum, which, accord- 

 ing to the lowest estimates, exceeds 1700°. The total surface of 

 the radiation was 385 square millims. ; but let us reckon only 3 

 square centims., in order to make a liberal allowance for the cir- 

 cumstance that the two extremities may be cooled a little by con- 

 tact with the electrodes ; and, finally, let us neglect the loss of heat 

 by contact with the air. Upon these data let us calculate the quan- 

 tity of heat emitted by this wire according to the law of Dulong 

 and Petit, taking the formula given by Pouillet, 



where e is the quantity of heat, one gram of water heated 1° being 

 taken as unit; g a constant, of which the value is 1*146 when the 

 square centimetre is taken for the unit of surface, and the minute 

 for the unit of time ; / is the emissive power ; a is the constant of 

 Dulong and Petit, or 1*0077; t the temperature. We will adopt 

 the number given for the emissive power by MM. La Provostave 

 and Desains,/= 0*092. 



On making the calculation, putting if = 1700, we find 48541 units 

 of heat, or 48*541 calories, per square centimetre of radiating sur- 

 face. For the whole of the wire, then, 145*623 calories should 

 have been evolved if the law of Dulong and Petit were applicable, 

 while the motor could supply only 42*3 at the most. The differ- 

 ence is enormous. 



Another method, capable of giving much more exact if not more 

 striking results, consists in taking a pile, a tangent-compass, and com- 

 pleting the circuit by a platinum wire of a certain diameter and a 



