412 Intelligence and Miscellaneous Articles. 



The ratio between the work expended and the intensity of the 

 current diminishes when the humidity augments — in such a manner 

 that, to obtain a current of given intensity, a greater velocity of 

 rotation is requisite in wet than in dry weather, but a less expen- 

 diture of work. The Holtz machine is therefore more economical in 

 wet weather than in dry. 



The distance between the two disks of the machine has also some 

 influence on the intensity of the current : the less the distance the 

 stronger the current, and the greater the amount of the work. 



The Holtz machine, like the voltaic couples, possesses electro- 

 motive force and internal resistance. The electromotive force is 

 independent of the velocity of rotation ; but it diminishes as the 

 degree of humidity increases. The effective motor weight (differ- 

 ence between the weights necessary to turn the machine charged 

 and not charged) is proportional to the electromotive force produced. 

 This is very great in comparison with the electromotive forces of 

 the most energetic voltaic couples ; in fact it has been found to 

 amount to 433000 Siemens units with a relative humidity equal to 

 0-69, to 599000 with the humidity 0-35— the Daniell couple giving 

 E =11*57, and that of Grove E = 19*98. The electromotive force 

 of the Holtz machine is therefore about 50000 times as great as that 

 of the Daniell couple, and 30000 times as great as that of the 

 Grove couple. 



The internal resistance of the Holtz machine is independent of the 

 hygrometric state, but varies with the rotation-velocity, diminishing 

 more rapidly than the velocity increases. It is very great : the 

 lowest resistance (which corresponds to the greatest velocity attain- 

 able, or 8 turns per second) is equal to 570 million Siemens units ; 

 for a velocity of 2 turns per second it is 2810 millions of the same 

 units. 



Under these conditions a resistance inserted in the outer circuit 

 must be very considerable in order to exert any sensible influence 

 on the intensity of the current. It was because the resistance em- 

 ployed by Poggeudorff was too feeble that he could not verify an 

 effect of this kind. M. Bossetti, on the contrary, by interposing 

 in the circuit a column of distilled water of greater or less length, 

 has ascertained that the current of the Holtz machine is susceptible 

 of being very notably weakened by augmentation of the external 

 resistance, following Ohm's law in this equally well with the ordi- 

 nary galvanic currents. 



The author has deduced from his experiments a measure of the 

 mechanical equivalent of heat, by comparing the serviceable work 

 expended for the production of the electricity with the total heat 

 which could be developed by the current obtained. He found for 

 that equivalent the number 428. — Bibliotheque Universelle, Archives 

 des Sciences, March 15, 1875, pp. 250-252. 



