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 XXXVIII. Intelligence and Miscellaneous Articles. 



ON THE DURATION OF INDUCTION-CURRENTS. 

 BY P. BLASERNA. 



TN the theory of induction-currents it has hitherto been generally 

 ■*- allowed — 



(1) That they are formed at the very instant the primary circuit 

 is closed or opened. 



(2) That their duration is infinitely small. 



I endeavoured to verify these two points experimentally by means 

 of a rotating apparatus, the construction of which is like that which 

 M. Guillemin used in his researches on the propagation of the cur- 

 rent of the battery, and also like that which M. Hipp used in his 

 experiments on the velocity of induction-currents in the great tele- 

 graphic lines. 



Two wooden cylinders, A and B, 8 centims. in diameter and 1 cen- 

 tim. in length, were fixed on the same axis, which could be rotated 

 with a velocity of from 20 to 150 turns in a second. On the circum- 

 ference of the cylinder A a brass plate was let in which occupied 

 about half the circumference (almost 180°) ; on that of the cylinder B 

 there was also a very thin brass plate which represented a degree of 

 the circumference. To vary the conditions, I had several spare cy- 

 linders with larger and smaller plates. To each cylinder there was 

 firmly fixed a cylinder (a, b) of much smaller diameter, made of box- 

 wood and entirely covered with brass. Thus the whole apparatus 

 consisted of the small cylinder a, of the cylinders A and B, and of the 

 small cylinder b arranged on the same axis, and with a good metaliic 

 contact between a and A, and between B and b. Four metal springs 

 rested against these four cylinders to break or make contact. They 

 were pressed by paper cushions, which stopped the vibrations. 



The primary current, furnished by several Bunsen's elements, 

 passed by a commutator, by the thick wire of a small induction-coil 

 (without a contact-breaker), through a tangent-compass, by the 

 springs of the cylinders a and A, and returned to the pile. It was 

 thus opened or closed according as the spring A rested on the wooden 

 cylinder or on the brass plate. The induction-current was then 

 formed in the coil, passed by the springs B and b, then through a 

 very sensitive galvanometer, and then back to the coil. It was 

 measured on the galvanometer. By varying the velocity of rotation, 

 and giving to the plate in B any position whatever behind that of A, 

 the mode of formation of induction-currents could be studied. 



I will mention the principal results I have obtained, intending 

 meanwhile to continue my researches with a much more perfect form 

 of apparatus, which I am now constructing. 



(1) The moment the primary current is opened or closed, an in- 

 duction-current is formed. The time which elapses between the 

 closing or opening of the principal current and the moment at which 

 the induced current is produced is so small that I have not been able 

 to determine it. In any case it is less than ^j^nr °^ a secon d. 



