76 Intelligence and Miscellaneous Articles. 



ON THE VARIATIONS OF THE RESISTANCE OF ELECTRICAL 

 MACHINES WITH THEIR VELOCITY. BY M. E. LACOINE. 



It is known that the resistance of magneto- and dynamoelectrical 

 machines is not constant, but increases with their velocity. The 

 experiments of which I am about to give an account to the Aca- 

 demy afford ground for thinking that these variations of the resis- 

 tance are to be explained by those of the contact between the 

 movable commutator and the rubber-springs. 



I employed a copper cylinder 5 centim. in diameter, with longi- 

 tudinal grooves resembling the insulating ones of a Gramme's com- 

 mutator. Upon this cylinder, at two diametrically opposite points, 

 rub two steel springs which form the terminations of the two ex- 

 tremities of a circuit comprising a battery, a telephone, and a 

 galvanometer to measure the resistances. The resistance of the 

 circuit was : — 



ohms. 



At rest 68 



With the velocity of 2000 rotations 183 



„ ' 4000 „ 900 



„ about 5000 „ 1567 



With, a very high velocity not measured 2900 



The sound given by the telephone was the louder and shriller 

 the greater the velocity. 



In this first series of experiments every spring touched the 

 cylinder over a space of three grooves ; on making them touch 

 over a greater extent (eight grooves), less sensible variations are 

 obtained. 



For each velocity the resistance diminishes when the pressure 

 of the spring is increased. It is possible even to reach a pressure 

 sufficient to suppress all microphonic effect and stop all sound in 

 the telephone. 



I have since modified the apparatus, and substituted for the 

 fluted cylinder one of the same diameter but with its surface 

 smooth. I obtained the same effects with a very little higher 

 velocity. Instead of making the springs touch on the cylindrical 

 portion, I caused them to press upon the plane lateral portion ; 

 the same results were observed. 



The measurements which I have taken give me reason to believe 

 that, for one and the same pressure of the springs, the increase of 

 the resistance is proportional to the cube of the velocity ; but it 

 would be necessary to repeat these experiments with means which 

 were wanting to me at Constantinople, in order to be assured of a 

 constant velocity and to measure it exactly. 



I will conclude by saving that with the telephone a more intense 

 sound is heard when the intensity of the current is less ; hence I 

 conclude that the sound given by a telephone placed in the circuit of 

 a Gramme machine is not due merely to the undulatorv variations 

 of the current, but also to the microphonic effect of the commu- 

 tator. — Comptes Rendus de VAcademie des Sciences, December 5, 

 1881, t. xciii. pp. 958, 959. 



