112 Mr. S. Bidwell. On the Changes produced by 



Each, turn would contain S'Srr cm. of wire (or rather more) = 

 12 cm. Therefore the number of turns would be 10,058/12 = 838. If 

 there were more than one layer of wire, the number of turns would be 

 fewer. The magnetising force would be nearly — 



4.f| C= 109C, 



C being the current in C.G.S. units. 



In Joule's experiments with stretched wires another coil was 

 used. 



Length of coil 11*5 in. = 28 '5 cm. 



Internal diameter .... 1 „ 



Length of wire 33 yds. =1188 in. 



Thickness of wire .... '1 in. 



The number of turns of wire would therefore be about 1188/1*1^ 

 = 344, and the magnetising force about 



4^0=1520, 



C being, as before, the current in C.Gr.S. units. 



Joule also describes his tangent galvanometer, and gives the deflec- 

 tion which the magnetising current produced in every case. The 

 galvanometer " consisted of a circle of thick copper wire 1 foot in 

 diameter, and a needle half an inch long furnished with an index." 

 The radius, therefore, was 6 inches = 15 2 cm., and the constant, Gr, 

 approximately 2^/15*2 = 0'41 .* The horizontal component of the earth's 

 magnetic force was, at the date of Joule's paper, about 0*17 ; thus 

 the factor by which the tangents of the angles of deflection should 

 be multiplied to give the deflecting currents in C.Gr.S. units is 

 0-l7/0-41=0'41. 



The greatest deflection recorded in Joule's experiments with iron 

 was 62° 48', the tangent of which is 1"95 ; the magnetising force was 

 therefore 



1 -95x0-41x109=87. 



The greatest deflection in his experiments with steel was 70°30' 

 = tan _1 2'824, the corresponding magnetising force being 126. 



The greatest galvanometer deflection in the experiments with 

 stretched wires was 61° 25', the tangent of which is 1'835, the cor- 

 responding current 0*75 C.G.S. units, and the magnetising force 114. 



Mayer used a coil 60"25 inches=153 cm. in length, the number of 

 turns being 1919. The magnetising force at the centre of his coil 

 1919 



was therefore about 47r-^-— C = 157'5C. 



* See Clerk Maxwell's " Electricity," vol. ii, p. 325. 



