224 
PROFESSOR J. A. EWING AND MR. W. LOW ON THE 
§ 5. The pole-pieces of the magnet used in the early experiments were only 5’2 5 cm. 
square, and it was clear that with a larger magnet a greater concentration of the 
induction might be produced. Professor Tait was kind enough to lend us the large 
magnet of the Edinburgh University Laboratory, and all the subsequent work has 
been done with it. 
The Edinburgh magnet has a pair of vertical limbs about 50 cm. long and 10'7 cm. 
in diameter. These are united by a horizontal yoke at the bottom, and are furnished 
on the top with movable pole-pieces in the form of rectangular blocks of soft wrought 
iron, the cross-section of which is 9'6 cm. square. The limbs are wound along a 
Pig. 3. 
length of 49 cm. with a number of coils which are grouped in series, making about 
1600 turns in all. The currents employed by us ranged up to 40 am 23 m’es, and the 
greatest value of the line-integral of the magnetic force was consequently about 80,000. 
lo allow the old bobbin, of the form of fig. 1, to be effectively used, we furnished the 
magnet poles with a pair of intermediate conical pieces of soft wrought iron, which 
virtually formed an extension of the bobbin’s ends. Fig. 3 is a full-size sketch of the 
poles a, a, with the intermediate pieces 1), h, and the bobbin c, in place. The figure 
shows the size to which the neck of the bobbin was finally turned down in experi¬ 
ments which are described below. The dimensions are given in millimetres. 
