Researches on Electro-magnetism. 201 



which swung a magnetic needle* (tangent needle), and was sent 

 thence through a wire which encircled an iron core. In the 

 neighbourhood of the latter was a second magnetic needle sus- 

 pended horizontally, so that the produced axis of the core struck 

 the central point of the needle, and was at right angles to its 

 meridian. Both needles were placed sufficiently far apart to 

 pre\'ent any mutual action between them. By means of Wheat- 

 stone's regulator, the strength of the current could be varied 

 within wide limits, and was measm-ed in all cases by the tangent 

 instrument. Every alteration in the strength of the cmi-ent 

 ought to be accompanied by a corresponding alteration of the 

 adjacent magnetic needle. If, now, these alterations were pro- 

 portional to each other, and if the angle which the tangent 

 needle, when acted upon by the stream circulating round it, 

 makes with the magnetic meridian be called <^, and the angle 

 which the excited core causes the other needle to make ^dth the 



same direction be called d>' then must the quotient 7* for the 



tan^' 



same series of experiments be a constant quantity. 



IMiiller assumes as unit for the galvanic power a stream which 

 produces a cubic centimetre of gas in a minute. To reduce the 

 angle given by the tangent instrument to this unit, the tangent 

 of that angle must be multiplied by 70. His smallest strength 

 of stream is 3-7, his greatest 63'5. The power which he applies 

 varies therefore between the limits of the ratio of 1 : 17. 



His iron cores were 560 millimetres long, and were encii'cled 

 by wire along their entire length. The number of windings on 

 every 10 millims. of the cores amounted, dm-ing one part of the 

 investigation, to 16 ; in most cases, however, it did not exceed 8. 



The magnetizing force stands nearly in the ratio compounded 

 of the strength of the stream and the number of windings which 

 act upon the same core. The greatest magnetizing force applied 

 by Miiller corresponded to the stream -strength 35-5, with 16 

 ■windings to every 10 millims., and hence exceeded in no case 

 the nmnber 568. 



Iron cores of considerable length, such as those used by 

 Miiller, are difficult to obtain free from coercive power. We 

 have therefore given the preference to shorter ones. The outer 

 suri'ace, whicli is generally rendered a little hard by hammering, 

 was turned off, and, in short, no pains were spared to obtain good 

 soft iron. Notwithstanding this, however, no core used by us 

 showed itself entirely free from magnetism after it had been once 



* We shall refer to this in future as the tangent needle ; and when taken 

 in toiniexion with the surrounding ring and the graduated circle, as the 

 tangent instrument. The ring of course stood perjjendicular to the ho- 

 rizon. — J. T. 



