PHYSICS: C. BARUS 
211 
AN ELECTRODYNAMOMETER USING THE VIBRATION 
TELESCOPE 
By Carl Barus 
Department of Physics, Brown University 
Communicated, April 21, 1919 
1. Introductory. — The employment of a telescope with a vibrating objective 
did good service as an aid to the interferometry of vibrating systems. It 
seemed worth while therefore to see what could be got out of it, when used in 
connection with a telephone only, as a dynamometer. The experiments are 
of interest both because of the vibratory phenomena observable and in view of 
the peculiar method of optic observation developed. Its possible use for find- 
ing the magnetic field within a helix of unknown constants deserves mention. 
2. Apparatus. — A front view (elevation) of the design is given in figure 1 
and an enlarged detail (side view) in figure 2. The apparatus consists of a 
rigid rectangular frame- work of \ inch gas pipe, A, B,C, D, EE', F being the 
foot attached to a tripod. There may be a steadying foot at C A and D 
are attached to EE' by the stout clamps c, c", so that EE' lying behind the 
plane of ABCD, admits of the attachment of a suitable clamp c', by which 
the telephone ih may be held in the same plane. B and C may be forced apart 
sHghtly by the screw n controlled by the broad thumb nut m, the conical end 
of n rotating in a socket of the cap p. 
The vibrating system consists of the bifilar wires of phosphor bronze dd' and 
the frame of the lens / which is the movable objective of the telescope T, the 
latter part containing the ocular and a plate micrometer (cm. divided in 100 
parts). T may be at a considerable distance (50 cm. or more) from/, and sup- 
ported by a convenient standard. The frame of the lens (which must hold it 
securely, cement being used if necessary) is of light sheet metal, the parts gg' 
being of sheet iron (about 0.05 cm. thick) so as to be attracted by the magnet, 
i, of the telephone. The stiff cross wires, r,s, of the frame are either soldered 
to the bifilar system dd' or otherwise attached to it (soft sealing wax does very 
well for temporary experimental purposes). 
The attachment and tension-control of the bifilar suspension is finally to be 
described, as its period must be synchronized with the alternating current. 
Results are obtainable only when the two periods are strictly in unison. In 
figure 1 the wires dd' are looped around a groove in the pipe D below, and the 
upper ends of dd' after passing a similar groove in A are bent around the post 
a, a', and wound respectively around the snugly fitting screws, b, b', the ends 
being secured against sliding by a fine hack saw cut in the screws. To stretch 
a wire it is passed from the notch in b once or twice around it, thence around a, 
downward by the groove to D and then up in the corresponding way to b'. 
