148 
Mr. A. Gray on the Determination of the 
tromagnets, and it depends on the action between a magnetic 
field and a conductor in the field carrying a constant current. 
The ends of the cores of the electromagnets were long straight 
surfaces of iron, separated from the surface of iron placed 
opposite to them by only a narrow space at each pole. In 
the experiments, of which I give some results below, the 
intensity of the field was found, in a space about 2 millim. 
wide, between one end of the iron core of an electromagnet 
and the surface of a thick piece of iron. The length of the 
pole-face was 30 centim. The magnet and corresponding 
iron were placed with their faces vertical ; and a copper wire 
was suspended vertically in the space between them, and in- 
sulated from their surfaces. The lower end of the wire dipped 
into a mercury-cup, and its upper end into a second mercury- 
cup on the top of the electromagnet, without touching the sides 
or bottoms of the cups. By means of these cups the wire 
could either be put in series with the coil of the electromagnet 
or in circuit with an independent battery or generator. The 
wire was hung in a vertical position (as shown in the diagram) 
from the lower end of a cord about two metres long, attached 
to a firm support above, and was kept taut by two half-pound 
weights (represented by the single weight W in the diagram) 
hung at the ends of a cross bar firmly attached to the wire. 
The position of the wire was adjusted by shifting the point of 
