396 USE OF THE GALVANOMETER 



ON THE USE OF THE GALVANOMETER AS A MEASURING 



INSTRUMENT. 



BY J. C. POGGENDORFF. 



[Translated from Pog. Anual. LVI, p. 324, by John D. Easter, Ph. D.] 



The galvanometers in use correspond very imperfectly with their 

 name, affording, as they do, a very uncertain and limited measure of 

 the force of the current. Even within the first ten or twenty degrees, 

 within which the deflection of the needle is usually assumed as pro- 

 portional to the force of the current, the accurate determination of 

 the relation between these elements is not so easy and simple, and 

 beyond these limits the problem becomes so complicated that even 

 its theoretical solution is extremely difficult. 



The determination is not absolutely impossible; it could be made 

 by Ampere's formula if all the requisite data, (length and form of the 

 coils, their position and distance from the needle, the size, shape, 

 and magnetic condition of the latter,) were given, but the calculation 

 would be extremely complicated and tedious. It would, even then, 

 hardly repay the trouble, for the result would still be unreliable, on 

 account of the probable errors in the determination of the data; and, 

 even if it were quite correct, it would only have a specific value, for 

 the calculation must be repeated for each instrument, and even for 

 each position of the needle in the same instrument. For this reason 

 no attempt has hitherto been made to form a theoretical scale of in- 

 tensity for the galvanometer, but various empirical methods have 

 been adopted, which, though they yield only special results, are 

 preferable, because less tedious, and therefore more easily repeated, 

 and capable of greater accuracy. 



Several such methods have been contrived by Becquerel, Nobili, 

 and Melloni. They all require the use of a series of currents com- 

 bined in various ways. With reference to this they may be divided 

 into two classes — methods by combination and by difference. 



The simplest of the former is given by Becquerel. He attached 

 to the galvanometer a thermo-pile, of which he set in action, succes- 

 sively, one, two, three, four, &c.,' pairs of plates, by heating each 

 alternate joint as uniformly as possible. According to the theory, 

 the force of the current must also increase in a ratio, one, two, three, 

 four, &c, and, consequently, the deflection of the needle correspond- 

 ing to these intensities may be read off directly. This need only be 

 marked on the instrument in order to establish the relation for all 

 future cases.* 



This method proceeds on the principle of passing, successively, 

 one, two, three, four, &c, currents of the same strength from the 



'-■ Traite" d' Electricity T. II, p. 24. 



