408 Intelligence and Miscellaneous Articles. 



method for the determination of the ohm. The method which I am 

 about to describe differs from that of the eminent physicist in not 

 requiring the quantities of heat to be measured or the mechanical 

 equivalent of heat E to be known. This last point is not unim- 

 portant ; for in Joule's calorimetric method the final approximation 

 is limited by the uncertainty at present existing respecting the 

 exact value of the number E ; that is to say, the possible error is 



near Tuir- 



The wire of which I wish to know the resistance is placed in the 

 middle of a vessel arranged as a calorimeter in the centre of an en- 

 closure with a constant temperature. An electric current is passed 

 into the wire, and its intensity i measured. I wait until, in con- 

 sequence of the heat liberated by the current, the vessel attains a 

 stationary temperature ; I leisurely ascertain that it is so by em- 

 ploving a thermometer, or, rather, a sensitive thermoscope, placed 

 inside the vessel. This done, I interrupt the current, and then set 

 in action a motor which produces friction in the midst of the vessel 

 that already contains the wire. The heat evolved by the friction is 

 substituted for that just before evolved by the current. I manage 

 so that the stationary temperature resumes its former value ; I then 

 have ri 2 =T, T being the work expended, whence the value of r. 

 It is scarcely necessary to add that the friction-apparatus must 

 remain in the vessel which contains it, even when it is not in ope- 

 ration, and be furnished with the known arrangements for measu- 

 ring T. It is also more convenient to commence by the friction- 

 experiment, and afterwards to regulate the intensity i so as to 

 recover the same stationary temperature. Lastly, it may be advan- 

 tageous, for apparatus of large capacity, to replace the observation 

 of the stationary temperature by that of the velocity of the heating. 



In the form given to it by Joule in 1867, the calorimetric 

 method of the English physicist rests equally upon the measure- 

 ment of i and the measurement of a mechanical work, namely the 

 work done at the time of the determination of E. Moreover, it 

 involves two calorimetric measurements, which are to be mutually 

 eliminated from the final result — namely, the calorimetric measure- 

 ment which accompanies the determination of E, and that which 

 accompanies the passage of the electric current. These interme- 

 diate determinations bring in their causes of error and their cor- 

 rections, owing to the imperfections of the calorimeters employed 

 in making them. I dispense with them by taking care to expend 

 the work T and the electric energy ri 2 in one and the same calo- 

 riscopic vessel. It becomes as needless to ascertain the quantity 

 of heat evolved in that vessel as to ascertain the weight of the 

 tare in a double weighing ; and the advantage obtained appears 

 analogous to that which there would be in replacing two successive 

 single weighings, made with different balances and different weights, 

 bv a Borda's double weighing. — Gomptes Rendus de VAcademit des 

 Sciences, Oct, 9, 1882, t. xcv. pp. 634, 635. 



