31. I. Pupin — Electrical Oscillations, etc. 325 



Art. XXX V III. — On Electrical Oscillations of Low Fre- 

 quency and their Resonance / by M. I. Pupin, Ph.D., 

 Columbia College. 



Part I. On the Production of Simple Harmonic Currents of 

 Constant Frequency by Electrical Resonance. 



The sensitiveness of the telephone for exceedingly small 

 alternating currents is well known. It is probably as great as 

 that of the most delicate Thomson galvanometer for direct 

 currents. Just as this instrument, so the telephone is especially 

 suited to zero-methods. But the telephone does not enjoy that 

 popularity in the precision room which its direct current rival, 

 the Thomson galvanometer, enjoys, although the field of phys- 

 ical research in which alternating currents must necessarily be 

 employed is very extensive indeed. The fault lies with our 

 alternating currents and not with the telephone. The alterna- 

 ting currents which the ordinary induction coil as employed in 

 physical laboratories produces is far from being a simple har- 

 monic current. The consequence is that in very many cases 

 the zero method, for which the telephone is especially suited, 

 has to be abandoned, and the minimum method substituted for 

 it, which, of course, is a poor substitution. 



Being engaged in a research in which I had to employ alter- 

 nating currents I tried, for reasons just given, to devise some 

 method of producing simple harmonic currents of constant 

 frequency, the frequency to be easily and very accurately deter- 

 minable. The following is my solution of this interesting 

 problem : 



A. On the Production of Alternating Currents of Constant 

 Easily and Accurately Determinable Frequency. 



My earliest solution of this problem consisted in producing 

 an alternating current in the secondary of a very small trans- 

 former by making and breaking very rapidly and at a constant 

 rate the primary. The interruptor of the primary current con- 

 sisted of the following arrangement : 



A stiff brass wire was stretched between the pole-pieces of a 

 permanent horse-shoe magnet. The wire was supported, just 

 as in a monochord, on two hard rubber bridges, and by varying 

 the distance between the bridges and the tension of the wire 

 it could be made to vibrate any note between about 60 and 

 1,000 complete vibrations per second. The middle part of the 

 wire was between the pole pieces of the permanent magnet 

 and carried just a short distance outside of the pole-pieces, a 

 short, thin amalgamated copper wire which dipped into a 

 mercury cup once during each vibration. At every dip it 



