104 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



weight of such a needle is easily kept within 0.75 gram and the air-damping is 

 quite sufficient. Unfortunately its period is large, being about i minute, and it 

 is apt to vibrate as a pendulum. Hence it is often convenient to hook on a wire 

 at t, bent like an inverted V, with the free ends submerged in water to secure 

 greater steadiness on the interferometer; or a mica vane may be added, as at w. 

 The bifilar suspension, 13.5 to 20.2 cm. long in the different experiments, 

 terminating above in the hooked brass rod r, is adjustably fixed in the brass 

 cylinder p, which in turn is secured in the hard-rubber insulator n, attached at 

 right angles to the brass standard GG, the lower end of which is screwed to the 



6 



FIG. 63. 



brass plate A A. This rod can be lengthened telescopically (not shown) ad- 

 mitting of different lengths of bifilar suspension. The hard-rubber lever 

 enables the observer to twist the bifilar. The charge, from a Zamboni cell 

 or the lighting circuit (250 volts), is conveyed to the needle through the hard- 

 rubber insulator at m and the clamp-screw at q (which in turn secures the 

 plug p), through the moistened bifilar wires, as in Dolezalek's apparatus; or 

 it may be admitted through the insulated damper below /. 



Finally, the lower part of the case CD envelops the quadrants more or 

 less permanently and is provided with wide plate-glass windows for observa- 

 tion. The upper part EF of the case may be taken off like a hat. 



68. Observations. Experiments were made with this apparatus at con- 

 siderable length, but they were not sufficiently definite to lead to any quanti- 

 tative statement. Great difficulty was experienced, in addition to the drift 

 of the needle, in securing an adjustment of the mirrors such that the beam of 



