623 
of Edinburgh, Session 1881 - 82 . 
entire instrument, wooden supports and carbons together, by the 
sound waves which strike against it and are thereby stopped. 
For distinction, we may call the first of these the air effect, and 
the second the tremor effect. 
In my experiments I have endeavoured to arrange the instru- 
ment so as to isolate these effects, and, as far as possible, examine 
each of them separately. 
To isolate the air-effect, it is obviously necessary, either to fix 
the carbons rigidly in their supports, so as to avoid any motion 
of the one relatively to the other, or to use a strong current, and 
place them just clear of contact with each other. 
The following experiment illustrates how this may be done • — 
Fig. 1. 
A, B and C (fig. 1) are three blocks of brass firmly fixed to a heavy 
wooden sole -plate S. To the top of A is soldered a piece of 
brass rod a about 2 inches long and § inch bore. To the top 
of B is soldered a piece of similar tube b about 4 inches long. 
Through C passes a fine screw worked by a milled head c. A 
piece of carbon rod e is fixed firmly into a , and has a hole \ inch 
wide drilled through its centre. A long piece of carbon / pointed 
at one end, passes tightly through the tube b , and can be moved 
backwards and forwards by the screw c. A piece of india-rubber 
tube is passed over the left end of the tube a and carries the 
mouth-piece M. By means of the wires g and h, soldered to 
the carbon rods, they are put in circuit with the battery (20 
Groves cells), and the telephone T, which must either have a 
small resistance or be placed in a separate circuit from that 
