70 Mr. W. Duddell [Feb. 21, 



It must not be forgotten that tbe arcs are ordinary direct current 

 arcs between solid carbon electrodes which are taking energy from 

 the direct current supply and converting it automatically without 

 any moving machinery into energy in the form of alternating currents 

 in the oscillatory circuit shunting the arcs. The frequency of this 

 alternating current can be easily varied over a very wide range. I 

 have already mentioned the range of two octaves which gave alternat- 

 ing currents having frequencies from 545 to about 2200 periods per 

 second, and higher frequencies up to a limit of about 10,000 periods 

 per second were easily obtained. No lower limit has yet been found, 

 frequencies down to 170 having been used, or a range of just under 

 six octaves. In order to produce the lowest frequency, a self- 

 induction of about • 15 henry and a capacity of six microfarads was 

 used ; under these circumstances it was found that although the arcs 

 were only supplied from 200-volt direct-current mains, the alter- 

 nating voltage between the terminals of the condenser sometimes 

 reached nearly 2000 volts. 



When the musical arc was first set up at the Central Technical 

 College, where all the experiments were originally devised, an 

 interesting observation in connection with the sensibility of the arc 

 as a telephone receiver was made. It was noticed that an arc in 

 Sir W. de W. Abney's laboratory played tunes, and this was found 

 to be due to the fact that my musical arc and the arc in Sir W. de 

 W. Abney's laboratory were supplied from the same street mains. 

 Although his arc was not specially adjusted to a sensitive condition, 

 and in spite of the two arcs being in separate buildings, 400 yards 

 apart in a straight line, and at a considerably greater distance if 

 measured along the mains, yet the sensibility of the arc was such 

 that whenever tunes were played on the musical arc, they could be 

 distinctly heard in Sir W. de W. Abney's laboratory. 



The conversion of direct current into alternating current by 

 means of the musical arc, besides being of scientific interest, forms a 

 very convenient means of obtaining alternating currents, with which 

 a large number of well-known experiments may be easily performed.* 

 It has, however, two further advantages, namely, that the frequency 

 can be easily adjusted to any required value, and that the frequency 

 is at once self-evident from the note the arc is giving out. 



The comparison of the capacity condensers and the self-induction 

 of coils can be easily made by comparing the pitch of tbe notes pro- 

 duced when they are used in the circuit shunting the arc. The 

 effect of closed secondaries and cores on the apparent self-induction 

 of coils is also very striking. The Elihu Thomson experiment of the 

 repulsion of a ring by a coil carrying an alternating current obtained 

 from the musical arc can be easily performed. 



If the primary of an induction coil (the contact maker being put 



* W. Peukert, Elektrotechnische Zeitschrift, 1901, p. 467. 



