622 Dr. W. M. Thornton on the 
applying the test to sample rings a time-constant of this 
magnitude was aimed at. 
Two rings were kindly prepared by Messrs. W. H. Allen, 
Son & Co., one of cast iron the other of cast steel *, both 
being the yoke-rings of standard machines with the brackets 
and facings turned off. The former was wound with 1000, 
the latter with 777 turns of copper wire to carry 50 amperes 
for a few seconds without undue heating, having respectively 
29 and 1°9 ohms resistance cold. In each case, however, 
the attempt to use the “make, short-circuit’ method, re- 
cording photographically, had to be abandoned on account 
of the difficulty experienced in obtaining satisfactory records. 
The motion of the spot of light was so rapid that the photo- 
eraph was too faint for accurate measurement. Another 
method had then to be found, and recourse was eventually 
had to the simple device of altering the current graduaily 
with several brief pauses, recording as before the induced 
voltage in the secondary or exploring coil. 
A vertical wooden pipe 10 inches square by 6 feet long 
was filled with dilute sulphuric acid and fitted with lead end- 
plates to which the voltage required to give the desired 
current was applied. A movable lead electrode was sus- 
pended from a pulley so that it could be readily raised or 
lowered. The movable and lower electrodes were connected 
through a standardized Weston ammeter to the. primary 
winding on the ring. With the movable plate at the bottom 
the winding was short-circuited, when at the top it received 
the full exciting voltage. 
The galvanometer used in the secondary circuit was by 
Pitkin, of the moving-coil type with the lower suspension 
made bifilar. It was just aperiodic. The recording drum, 
5 inches diameter by 10 inches long, was contained in a box 
provided with an adjustable shutter, and was at first driven 
by a clock within the drum. It made one revolution a 
minute and ran for ten minutes, the speed slowly decreasing 
from the start. 
To avoid this drop of speed a small spindle was screwed 
into the end of the axle of the drum, and, passing through 
the end of the box, was fitted with a pulley so that by using 
a gramophone-clock as motor a suitable constant speed could 
be obtained for the record. 
In applying the magnetizing current the pauses, which 
were about every ten amperes, were always long enough for 
the spot of light to remain for a moment at rest so that the 
curves as recorded were as follows (fig. 1). 
* Vide Journal Inst. of Elect. Engrs. vol. xxxiii. p. 539. 
