1911-12.] Transverse Induction Changes in Demagnetised Iron. 299 
require to be made on the calculated value of the demagnetisation angle 0 
due to the combined fields. 
Figs. 3 and 3 a show the arrangements in further detail and are drawn 
to scale. The former is a sectional elevation, the external connections 
being diagrammatic only. Y is a massive Hopkinson yoke of annealed 
Lowmoor iron, split in two. The experimental steel tube S T rests on the 
under half, while the upper half of the yoke secures it in position by its 
own weight. The hole in the double yoke was bored true and fits the steel 
tube accurately. Two small holes were drilled in the shell of the tube on 
a line parallel to its axis, each 2 - 6 cms. from the centre, through which 
10 turns of fine wire were threaded to form an exploring coil for measuring 
the circular induction. A pasteboard cylinder fits loosely over the steel 
tube. Two exploring coils of fine wire and two layers of copper strip 
forming a solenoid were wound on the former. The first consists of 30 turns 
for the purpose of measuring the longitudinal induction B L due to the 
solenoid producing the longitudinal field H L . This coil of comparatively few 
turns is only used when a current is flowing in the solenoid and measures 
not only the longitudinal induction in the steel tube, but the longitudinal 
field in the air space which it encloses. The ratio of the sectional area of 
this space to that enclosed by the tube is 42 : 30, and, as it is relatively large 
in comparison with the sectional area of the shell of the steel tube, a 
compensating coil A A of 42 turns was introduced within and secured to the 
inside of the tube. These two coils are in the same plane and, being con- 
