635 
of Edinburgh, Session 1877 - 78 . 
the pitch or musical sound that generally accompanies it. When, 
however, a prolonged vibration is difficult, and the impulses brace 
each other up by frequent repetition, we have possibly in the first 
case a very short series, and in the other only one vibration. Such 
vibrations are as capable of rendering all complex acoustic combina- 
tions as vibrations of the push-and-pull kind. 
It would be a matter of mere speculation to guess how the condi- 
tions of the vibrating helix are transferred to vibrating rods or discs 
of iron in an intermittent magnetic field. I would only say that 
the same double vibration is clearly traceable in them. To illus- 
trate this in the case of rods, I took a small coil of ISTo. 20 wire, 2 
inches in diameter and about an inch high with a hollow axis of 
| inch, and sent the interrupted current of a five Bunsen cell bat- 
tery through it. Inside the axis I put a soft iron pin 2 inches long 
and \ inch square (fig. 10). To the upper end a fine copper wire 
was soldered to act as the thread of a telephone. When rightly 
placed the pin supported itself in the hollow, and kept dancing up 
and down symmetrically without much friction against the inside 
of the bobbin hollow. Here the mechanical motion was not so 
clearly eliminated as in the case of the helix ; yet the ticking was 
heard, and it alone, when the motion of the pin was stopped by the 
hand. The impossibility of stopping the ticking of the pin was 
Fig. 10. F 
shown by securing its ends between the jaws of a vice and making it 
as tight as possible, when the vice itself took up the tale of electric 
interruption, and made itself heard all round. A curious change 
was observed in long iron rods when this coil was placed round the 
middle of them and when shifted to the end. In the former posi- 
tion the sound was a stuccato rendering of the longitudinal note of 
the rod, and in the latter this sound was lost in a dull tapping. In 
