64 Mr. W. Duddell [Feb. 21, 



WEEKLY EVENING MEETING, 



Friday, February 21, 1902. 



His Grace The Duke of Northumberland, E.G. D.C.L. F.R.S., 

 President, in the Chair. 



W. Duddell, Esq. 



Musical and Talking Electric Arcs. 



It is almost exactly a century since the discovery, and first exhibition 

 of the electric arc in the lecture theatre of the Royal Institution by 

 Sir Humphry Davy. During these hundred years attention has 

 been chiefly centred on the light given out by the arc and on its 

 practical utilisation. This evening, however, I will treat more par- 

 ticularly of the sounds it can produce. It is only of late years that 

 any interest has been taken in these sounds, and this interest has 

 chiefly originated from the work of Mrs. Ayrton, who classified and 

 described the noises which arcs produce under different conditions of 

 working. 



The sounds emitted by electric arcs can be divided into two 

 classes — those produced spontaneously, and those due to outside causes 

 acting on the arc itself or on the circuit supplying it. In cither case 

 the sound waves seem due to the rapid variations in the volume of 

 the vapour column which exists between the two electrodes. If the 

 current through any direct current arc be increased or decreased 

 slowly, and if the vapour column, or better still, its image projected 

 on a screen be examined, it will be at once apparent that an increase 

 in the current flowing through the arc is accompanied by an increase 

 in the size of the vapour column, and vice versa. Let the current be 

 periodically increased and decreased, and the vapour column will also 

 expand and contract periodically, and if the variations in the current 

 and the consequent periodic expansion and contraction of the vapour 

 occurs \^ith sufficient frequency, audible sounds are produced. 

 This can easily be demonstrated either by causing part of the direct 

 current which flows through the arc to pass through and operate an 

 electrically driven tuning-fork, or by adding to the direct current 

 flowing through the arc an alternating current. 



The sensibility of the arc to small variations in the current sup- 

 plying it is really surprising ; for instance, the small fluctuations in 

 the current supplied by a direct current dynamo, caused by the com- 

 mutator segments passing under the brushes, can generally be dis- 

 tinctly heard in any arcs supplied by it. A Wehnelt or other 

 powerful interrupter connected to the street mains will cause any 



