of Palladium caused by Absorbed Hydrogen. 137 



"wound, and (2) after the coil was wound, the ends being 

 held out in a straight line. From the length thus found of 

 wire wound on, the total area of the coil can be deduced 

 thus : — 



If I be the length of the wire wound, n the number of 

 turns, A the total area, we have, to a close approximation, 

 if the normal to the wire always makes a small angle with 

 the radius, the bar denoting mean value, 



_ _ I 2 



A = nirr' 2 = mrr 2 = -. — . 



4ft7T 



The materials of this coil (either the copper wire or the 

 seccotine or both) proved to be badly chosen, the coil being 

 almost as strongly paramagnetic as the uncharged palladium. 



6. Measurement of the Magnetic Force. 



As already mentioned, a similar coil B was wound, which 

 was used as an exploring-coil to measure the value of the 

 iield at a certain spot in the usual way. The coil was waxed 

 into a piece of ebonite fixed to the end of a light lever-arm, 

 weighted just beyond the fulcrum, which, when released, 

 raised the coil quickly out of the field. A change in the 

 rate of raising the arm was found to make no difference to 

 ■the galvanometer-throw, and it was therefore assumed that 

 the movement was rapid enough to give correct readings. 

 A piece of plate-glass was wedged flat against the lip of the 

 concave pole-piece, and the ebonite piece containing the coil 

 slid against this, so that the coil came accurately back to the 

 same position each time. This position was so chosen that 

 the coil was coaxial with the field and halfway between the 

 pole-pieces, and was, as nearly as could be judged, the same 

 as that afterwards occupied by coil A. Measurements of 

 the field were then taken for four values of the current from 

 1*7 to 7 amperes. The current for each value, before 

 measurements were taken, was reversed 50 times each way, 

 after which the throw was constant, and the same for both 

 •directions of the current. The flux through the coil when 

 raised out of the field was found, by breaking the exciting 

 circuit, to be negligible. 



In the galvanometer-circuit were included the coils of a 

 Hibbert standard inductor (and for the stronger fields used, 

 an extra resistance) and the galvanometer was thus stan- 

 dardized for each value of the resistance. The strength of 

 the field at one particular spot is thus found for all the 

 values of the current for the steady state of the magnet. 



