152, Mr. A. Gray on tJie Determination of the 
trical on the two sides of the knife-edges, and is horizontal 
when the knife-edges are turned downwards. The sides on 
which the knife-edges are placed may conveniently be slightly 
bent so as to have an upward convexity, in order that the 
line of knife-edges may, when the wire frame is made to rest 
on them, be a little above the centre of inertia of the system, 
in order that the frame may rest stably in the horizontal posi- 
tion ; or the frame may be loaded so as to have sufficient sta- 
bility by pieces attached below the knife-edges. The ends of 
the wire are to be bent first a little up, then downwards so 
that the point of one is about an inch nearer to the frame 
than the other, and both points are nearly in the line through 
the knife-edges. 
The field-magnets are set up so that the plane cutting their 
pole-faces in which the field-intensity is to be measured is 
horizontal ; and two supports, carrying metal Y-shaped rests 
for the knife-edges, are arranged so that the frame when rest- 
ing on its knife-edges has the two sides lying along the pole- 
faces in the horizontal plane through the two parts of these 
faces near which the field is to be determined. Two cups 
containing mercury are arranged on a support so placed that 
the ends of the wire of the frame dip into the mercury, with- 
out touching the sides or bottoms of the cups. One terminal 
of a convenient battery is connected to one of the cups ; and 
the other terminal is so arranged that the circuit can be com- 
pleted by depressing the terminal into the other cup, or broken 
by raising it, when required. 
When an experiment is made, the magnets are excited by a 
current of the required amount, and a measured current is sent 
round the frame, which is made to turn on its knife-edges by 
the electromagnetic action. A weight which can be slided 
along the frame is hung on one of the sides which carry the 
knife-edges, and moved to such a distance from the line of 
knife-edges that the frame is brought back to its initial posi- 
tion. This distance in centimetres and the mass of the weight 
in grammes are carefully determined, with the strength of the 
current flowing in the frame ; and the mean intensity of the 
field over the space occupied by the part of the frame in the 
field calculated from the results. Let I be this mean intensity, 
L the length of wire of the frame in the field, b the horizontal 
distance in centimetres of each side from the line of knife- 
edges, W the mass of the equilibrating weight in grammes, 
and r its distance from the line of knife-edges, g the accelera- 
tion produced by gravity (= 981 centim. per second per 
second), and 7 the strength of the current in C.G.S. units ; we 
have by equating moments, 
T Wr (7\ 
