204 MEASUREMENT OF CURRENTS. 





CHAPTER II. 



MEASUREMENT OF CURRENTS. 



826. DIFFERENT METHODS. The intensity of currents may be 

 estimated either by a direct measurement of the yield (824) in the 

 form of successive sparks, or by any of the physical effects which 

 accompany it, such as the heating of a conductor in conformity 

 with Joule's law, the local heating of a soldering, chemical action, 

 etc. ; but use is more especially made of the electromagnetic pro- 

 perties of currents, which give methods of observation which are 

 more rapid and capable of greater accuracy. 



The intensity at a point in the magnetic field of a coil traversed 

 by a current is proportional to the strength of the current. The 

 measurement of the current is thus reduced to a measurement 

 of the strength of the field. 



Galvanometers are instruments in which the field is measured 

 by its action on a magnetised needle. Both the idea and the name 

 of galvanometer are due to Ampere.* Schweiggerf devised the use 

 of the multiplier that is to say, a coil which multiplies the strength 

 of the field or the action of the current on the needle. 



The magnetic moment of a coil is equal to the product of the 

 strength of the current by the total surface more exactly, by the 

 sum of the projections on the mean plane of the coil of the surfaces 

 comprised by the different windings. We might thus also deduce 

 the intensity of a current from the action exerted by a magnetic 

 field on a movable coil. 



We may finally cause a fixed current, traversed by a current I, 

 to act on a movable current I' which is or is not different from the 

 first. The action is proportional to the product of the intensities IT, 

 and may serve to measure either of them. Instruments constructed 

 on this principle are called electrodynamometers. 



* AMPERE. Ann. de Chim. et de Phys. [2], Vol. xv., p. 59. 1820. 

 t SCHVVEIGGER. Allgemeine Litteraturzeitung, No. 296. Nov., 1820. 



