194 
DR. E. TAYLOR JONES ON THE RELATION BETWEEN 
support S above the coil, and the tube projecting slightly from the coil below. 
A thin brass rod R was screwed into the block B, and passing downwards through 
an opening in the lower end of the tube T, was attached to a scalepan below. 
The method adopted for observing changes in the length of the nickel wire was 
essentially the same as has been used by several experimenters. At one end of a 
long light lever L, of wood, a sharp gun-metal point pressed upwards against the 
lower end of the tube T, and the fulcrum F consisted of two gun-metal points 
resting on two plates of glass fixed on a support which was rigidly connected with 
the upper support S; the connexion is not shown in the diagram (fig. 1). At the 
end of the other (longer) arm of the lever, a small metal plate was attached, at one 
point of which a slight depression was made; in this depression rested one of the 
three needle-point feet of a small light table on which a plane mirror was mounted. 
The two other feet rested on a fixed horizontal metallic support. Deflections of this 
mirror were observed by a telescope and vertical scale at a distance of about 
3 metres. 
Levers of various lengths were tried ; the arms of the lever ultimately chosen were 
3‘275 centims. and 60’92 centims. long, measured from the hue joining the fulcrum 
points to the point at one end, and the depression at the other, respectively. The 
distance of the moving foot of the mirror table from the line joining the two flxed 
feet was 8*97 millims. The magnifying power of this arrangement Avas 12,300. 
The nickel wire was thus suspended inside but independently of the coil. The 
effect of any “suction” between the coil and the wire was, however, quite negligible, 
on account of the great rigidity of the support S, and becamse this support was 
rigidly connected with the fulcrum support of the lever, so that any displacement of 
the ujAper support involved a similar displacement of the fulcrum support, and 
therefore no turning of the lever. This was tested by observing the scale deflections 
caused by placing weights on the upper support S. Even if the wire had only one 
pole, the error due to this cause would be less than 0*1 per cent, of the deflection 
observed when the current Avas made. 
The error due to the slight non-uniformity of the field of the coil towards the ends 
of the nickel wire Avas also of the same order. 
The magnetising current Avas generally measured by a Kelvin graded galAmnometer, 
standardised by electrolysis of copper, the smaller currents by a Kelvin centiampere 
balance. 
For measuring the magnetisation, which Avas done in separate experiments, 600 
turns of No. 40 double-silk coAmred and shellacked copper AAu're AA'ere Avound in one 
layer near the middle of the nickel Avire. These Avere connected to a ballistic 
galvanometer Avhich Avas Avound Avith wire calculated to give the greatest sensitive¬ 
ness,* A long solenoid with secondary coil was used to standardize this galvanometer 
in the usual AAmy. 
* See A. G-ray, “ Absolute measurements in Electricity and Magnetism,” vol. 2, Part II., p. 369. 
