PHYSICS: C. BARUS 
331 
4. Bifilar systems. — To utilize such a system as figure 1 to advantage it 
would be necessary to attune the elastic vibrators at m, m', to the same period, 
which should be as nearly as possible identical with the period of the source of 
intermittent or alternating current. As an earth inductor or a small magneto 
inductor (single magnet rotating in a flat coil) would have to be used in the 
latter case, it seemed best to convert the apparatus into a bifilar vibrator as 
shown in figures 3 and 4 in elevation and plan. Here mm' is a strip of thin 
mirror plate glass, about 32 cm. long, 1 cm. broad and 2 mm. thick, horizontal 
and in a position to receive the rays N, N' of the interferometer (compare 
figure 1). Motion of mm parallel to itself, fore and aft, will therefore produce 
no effect on the fringes; but any rotation around a vertical axis will be im- 
mediately apparent. 
This strip of glass is supported by the bifilar system e e e' e', made of a single 
thin wire. The ends of ee' are wound around the horizontal screws a, b which 
rotate with friction and are supplied with an index and scale, so that any amount 
of tension may be imparted to the wire. This passes below under the pulleys c, 
d, as nearly free from friction as possible, with the object of securing the same 
tensions throughout e e'. Flat clamps/, f of fibre and screws, attach the strip 
mm' to the wire at any height, but necessarily near the middle of the vertical 
threads, where it receives the rays N N'. 
The telephonic system consists of the horizontal soft iron screws h, h' simi- 
larly attached to mm' by the flat fibre clamps g, g', and the telephones /, t'. 
These were made of large flat files. The screws / 1' are used to approach the 
telephone magnets /, t', as near the soft iron armatures h, h' as possible, without 
overstepping the unstable position, in view of the tension of the tense wire 
e, e'. To give the vibrating system adequate damping, thin wires q q' less that 
a millimeter thick, bent and dipping into lubricating oil in a small vat p, p' 
suffice. The fibres e, e' were about 45 cm. long and their distance apart about 
29 cm. Their period and that of the vibrating telescope were made about the 
same, on the average about 0.2 seconds. 
As a generator an earth inductor with a coil of wire 60 cm. in diameter was at 
first used but later replaced by a small magnetic inductor. It was turned by a 
small motor. To measure the average intensity of current a Siemens pre- 
cision dynamometer was installed, indicating currents as low as one-tenth of 
an average milliampere. Periods from about 0.1 second to 0.3 second were 
available. 
The three vibrating systems (mirror, telescope, alternator) thus all admit of 
an adjustment of their periods and these should be nearly the same if the ellip- 
tic system of Lissajous curves are to be obtained, which is the preferable case. 
A change of the tension of the wires e e' in figures 3 and 4, or any adjustments 
at the telephones calls for a fresh search for fringes; but this is not difficult. 
5. Further observations. — Without synchronism in the two vibrating systems 
(current and telephone) the motion of fringes obtained is practically inappre- 
