446 Horizontal Component of the Earth' 5 Magnetism at Oxford. 



We may dissect this quantity roughly by observing that, 

 if the steel have a permeability of about 30, which is a value 

 such as is believed to occur, the resistance of the length of 

 each bar (20 centim. nearly) would be about fg or *66 

 centim. The resistance due to the shape is probably about # 2, 

 though this is a matter I have not gone into with square bars ; 

 but if this is so, the permeability might be more like 40, 

 so that *5 centim. resistance would be due to the metal and 

 '2 to the shape. Observe the practical importance of this. 

 What a thing it would be to be able to tell beforehand, ever 

 so roughly, what the coefficient of induction of a magnet will 

 be. It is with a view to such practical objects that I have 

 introduced the idea of magnetic resistance. 



The dip can be obtained by these inductive methods so as 

 to give the vertical component when the horizontal is known, 

 with sufficient accuracy for practical purposes. The ratio 



. — = — r of the components is the tangent of the dip. 



horizontal x 



I have a coil wound round a wooden circle. This is placed, 

 first so as to cut the horizontal lines of force twice when 

 turned through half a revolution, secondly so as to cut the 

 vertical lines of force twice when turned through half a revo- 

 lution. The ratio of the throws of the ballistic galvanometer 

 in the two cases is the ratio required. 



The deflections obtained in such a determination were 913' 

 and 374' respectively. The ratio of these is tan 67° 43 /# 4, 

 which may be taken to be the observed dip. 



The vertical force must be obtained from the horizontal by 

 multiplying by tan dip. As I have no determination of H 

 on the day on which the dip was observed, I must take the 

 mean value '18010, whence the vertical force is found to be 

 •43968. 



The comparison of these numbers with the Greenwich 

 values and those deduced from Gauss's table is as follows : — 



Dip. Vertical force. 



Greenwich 1884 .67° 24'-0 "43686 



Correction from Gauss's differences. + 27' , 2 



Values thus deduced 67° 51'-2 -44125 



Observation 67° 43'-4 '43968 



The Gauss Greenwich vertical force is deduced from the 

 value of H determined by his differences, and the dip also so 



