Wadsworth — Interrupter for Large Induction Coils. 497 



For these last reasons, and also on account of the rapid oxi- 

 dation of the surface, even when covered with alcohol, such 

 contacts are entirely unsuitable when exact equality of the 

 successive discharge intervals is of importance. Another 

 objection to this form of interrupter, which becomes a serious 

 one when work requiring concentrated attention on the part 

 of the observer is under way, is the noisiness of its operation. 



About four years ago I designed a form of interrupter for 

 Prof. Michelson, which has been used ever since for the large 

 induction coil employed to excite the Geissler tubes containing 

 the vapor of the substance whose radiations are to be examined 

 by means of the wave comparer.* The conditions which the 

 interrupter is here required to fulfill are unusually severe. In 

 the first place the action should be as noiseless as possible, in 

 order to avoid distracting the attention of the observer. It 

 must be constructed so that it will run constantly, often for 

 some hours, without any attention or readjustment, even when 

 the current in the primary is varied from zero to its maximum 

 value (15 to 20 amperes), and finally in order to secure a per- 

 fectly steady source of light, the successive discharges should 

 be as regular as possible, both as regards intensity and dura- 

 tion. For certain portions of the work it was also essential 

 that the spark interval should be very uniform and considerably 

 less than is usually obtained with a spring interrupter whose 

 amplitude of swing is sufficient to enable it to be used with 

 currents of any magnitude. It was also desirable that the 

 number of breaks per second might be varied within considerable 

 limits without interfering with the action of the interrupter, 

 and that the observer might be able to readily start or stop the 

 latter without leaving his seat. 



As all of these requirements have been satisfactorily met in 

 the interrupter above referred to, and as it is, moreover, of 

 very simple and inexpensive construction, it was thought that 

 a brief description of it might be of interest to the spectro- 

 scopists and others who have had to use large coils for similar 

 purposes. 



It consists simply of a brass wheel about Q" in diameter, with 

 two insulating and two contact segments symmetrically placed 

 in its circumference, and mounted directly on the shaft of a 

 small electric motor making about 1200 revolutions per minute. 

 Two copper brushes are arranged to bear, one on the hub of the 

 wheel, the other on its circumference. A plan view of the 

 arrangement, showing also the electric connections with the 

 coil, is shown in fig. L, where M is the motor (for an inter- 

 rupter of this size a yj- to ^ -H-. P. motor is quite sufficient), 



*See paper, Application of Interference Methods to Spectroscopic Measure- 

 ments. II. A. A. Michelson. Phil. Magazine, vol. xxxiv, p. 280, Sept., 1892. 



