26 



DISPLACEMENT INTERFEROMETRY BY 



against this that the conical end of the iron rod rr to be examined pushes. 

 The spring 5 attached to the blade d and a lateral projection from the fork 

 F (or other appurtenance) insures continuous contact at a constant pressure. 



The iron rod rr, about 43 cm. long and 0.67 cm. in diameter in the first experi- 

 ment, is enveloped by a tubular water-jacket ww, i .40 cm. in external diameter. 

 Through this a current of water entering at t and leaving at t' is kept flowing 

 from a large copper Mariotte flask about 50 cm. high and 30 cm. in diameter. 

 The water contained is at the temperature of the room. Two such flasks were 

 at hand to be used alternately. The magnetizing coil CC, 26 cm. long and 3.7 

 cm. in external diameter, is wound immediately on the tubular water-jacket. 



The rod rr fits the tube ww loosely and is centrally attached at the remote 

 end by aid of a bushing 5 and a small bolt u. The conical front end is free. 



^-^l^^u) 

 T->cu J- 



m'. 



i' 



- 



The coil CC is held in position by a large clutch (not shown) encircling it at 

 the middle and attached to the bed-plate of the interferometer. It is addi- 

 tionally attached at the tubulures i and /'. Finally the conical end M of a 

 micrometer-screw (also rigidly attached to the bed plate) gives the remote 

 end of the rod rr any desirable fiducial position. This micrometer M has the 

 further advantage of permitting an independent corroborative standardization 

 of the contact lever, as there would obviously always be sufficient elastic 

 yielding in the apparatus to considerably shift the interference fringes; but 

 it is better to free the rod from the coil at the rear end, also. 



In the experiments made, the breadth of the ray rectangle mm' of the inter- 

 ferometer was 6 = 9.7 cm.; the normal distance between the rod rr and the 

 axis a, 7.0 cm.; the length of the contact lever a to mm', 10.6 cm.; and the 

 length of the axle a, 10 cm. 



The rod rr may be easily withdrawn and others inserted. 



