326 



M. I. Pupin — E lectrical Oscillations of 



closed the circuit of a gravity cell and the action of repulsion 

 between the current now flowing through the stretched brass 

 wire and the poles of the permanent magnet kept up the vibra- 

 tion of the wire when once started. In fact, when well ad- 

 justed the wire would start to vibrate of itself, making the 

 primary current at every downward stroke and breaking it at 

 every upward excursion. The current in the secondary was 

 an alternating current, of course, of exactly the same frequency 

 as the vibration of the wire. The frequency of the vibrating 

 wire could be varied by varying the tension gradually until 

 the vibration of the wire was in exact unison with a standard 

 tuning fork. Varying the tension (in a manner which will be 

 described below) of the brass wire did not interfere with its 

 vibration, so that the tuning could be made very accurately by 

 watching the beats. In this form, this what I call electrody- 

 namic interruptor, was shown to Professors Abbe, Barker, 

 Mendenhall, Michelson, and Rowland, during the autumn meet- 

 ing of the National Academy of Sciences in New York, in 1891, 

 and was very favorably commented upon by these scientists. 



In the meantime experience suggested the form given in 

 fig. 1, as best suited to the purpose for which the interruptor 



Fig. 1. 



Fig. fr 



h-^K 



2 



3: 



was first designed. The diagram of fig. 2 explains the con- 

 struction of the apparatus more clearly. A stout aluminium, 

 or phosphor-bronze wire, the vibrator, is stretched between the 

 polepieces d, and e, of two permanent Weston magnets, such 

 Pig 3 as this distinguished electrician uses in his 



voltmeters. 



Fig. 3 gives the front view of one of the 

 magnets. The cross section of the vibrator is 

 seen there between the polepieces 1ST S as a 

 black dot. The short line, a, b, extending 

 from the vibrator to the mercury cup below 

 is the dipper, a short, thin amalgamated cop- 

 per wire, which is soldered to the vibrator. 

 The vibrator rests on two hard rubber bridges, 



