870 
ture was wound in its middle part with 40 turns of copper wire, the 
ends of which were connected by means of two large spiral wind- 
ings with a pair of fixed terminals, in such a way as not to 
hamper its vibrations. If the interrupter is connected into a circuit 
with an inductionless ballastresistance of abont 1 Ohm and with 
two accumulator cells, the vibrations have an amplitude such as to 
render the distance of the armature from the cores taken together, 
variable from 2 millimeters to 7.6 millimeters. Without current the 
sum of the airgaps has a length of 4.8 millimeter. The selfinduc- 
tion of the electromagnet, which of course is not constant, has 
during the passage of the current a mean vaiue of about 9.3 
millihenry. 
Whilst the interrupter was in action, oscillograms were taken of 
the current through the electromagnet and at the same time the 
magnetic density in the armature was oscillographically recorded. 
For the current a high frequency DuppeLL oscillograph of the Cam- 
bridge Instrument Cy was used, whilst the magnetic density was 
recorded with a Sremens and Hatske oscillograph, or with a striug 
galvanometer. On the oseillographic records time marks of 0.01 
second were inscribed. For the stringgalvanometer records 0.001 second 
marks were used. 
