298 MEASUREMENT OF CURRENTS. 



time /, and then set up an instantaneous connection, lasting about 

 0*0002 seconds, between a galvanometer and two points of the 

 circuit. The needle of the galvanometer experiences a swing at 

 each contact; but if the cylinder turns continuously, it acquires a 

 permanent deflection proportional to the principal current. M. Guil- 

 lemin found thus that on air lines from 280 to 1000 kilometres in 

 length, the time necessary for setting up definite current varied from 

 0*004 to 0*028 seconds. The results depend, moreover, on the energy 

 of the battery, and more especially on the condition of the line. 



M. Blaserna* used an analogous commutator to observe the 

 extra currents which are produced on opening or closing the battery. 

 The intensity of the principal current at a given time was deduced 

 from the quantity of electricity induced in an adjacent circuit on 

 breaking (907). As the two effects reproduce themselves periodi- 

 cally, the galvanometer of the inducing circuit and the ballistic gal- 

 vanometer of the induced circuit both gave permanent deflections. 



Bernstein f and MoutonJ applied the same method to the in- 

 vestigation of electrical oscillations produced by induction in an 

 open circuit. 



In the apparatus of Bernstein, a disc, rotating about a vertical axis, 

 has four points united in pairs and situated on the same diameter. 

 The former two dip in mercury cups in the form of sectors, close 

 the inducing circuit, and break it again after an interval long enough 

 for the permanent regime to have become established. The ends of 

 the induced wire are connected by a galvanometer of high resistance 

 on the one hand with a continuous ferrule near the axis, in which 

 dips the third point ; and on the other with a stretched iron wire, 

 which the latter point grazes for an instant at each turn of the disc. 

 A micrometric screw, which acts on the wire, enables us to estimate 

 the time which elapses between this contact and the rupture of the 

 inducing circuit. The needle of the galvanometer tends to acquire 

 a permanent deflection when the connections are made. Bernstein 

 observed the first swing, and did not wait for the state of equi- 

 librium. 



Mouton made use of three wheels A, B, C, mounted on the 

 same axis and turning with a uniform motion. The first A closes 



* BLASERNA. Journ. des Sc. Nat. et Econom. de Palerme, Vol. VI.", 9. I. 

 !87o. Ann. de Chim. et de Phys. [4], Vol. XXIL, p. 500. 



+ BERNSTEIN. Pogg. Ann., Vol. CXLII., p. 54. 1871. 



i MOUTON. Ann. de VEcole Normale [2], Vol. VI., p. 207. 1877. 

 Journal de Physique^ Vol. VI., pp. 5, 46. 



