28 DISPLACEMENT INTERFEROMETRY BY 



The results obtained with these small fringes are given in figure 30, where 

 the abscissas show the current i passing through the helix and the ordinates 

 the corresponding fringe displacements, Ae, in the ocular. 



Owing to tremors of the laboratory and possibly also to actual instabilities 

 connected with the magnetization, the results are not smooth. But such 

 disturbances are secondary. As a test of the functioning of the contact lever 

 the data are very satisfactory. Thus it was quite possible to ascertain Ae = o.i, 

 i.e., elongations of but A/// 5Xio- 8 , equivalent to A/ = 2.3Xio~ 5 cm. It is 

 interesting, therefore, to note that the current must exceed 0.02 ampere 

 before any elongation can be detected. After this, however, the elongations 

 abruptly begin and increase rapidly to a maximum, which is reached before 

 saturation. They then decrease somewhat more slowly and eventually become 

 negative. In the strong fields the contact lever is thrown into violent vibra- 

 tion on closing the circuit, and the reading is less certain. 



The next experiments, figure 31, were made with somewhat greater care 

 and with larger fringes. The standardization of the ocular micrometer showed 



A=5.sXio 4 AvVcm. or( ) =10-^X1.82 



\AeJ 



Hence 



A///=io- 7 X2.i6Ae 



the data being otherwise the same as above. Here 



A/= io- 6 Xp.3A^ cm. 



The results in figure 31 are smoother than in figure 30, and but for the 

 incidental difficulties mentioned they would probably be quite smooth. The 

 character of both curves is about the same as to maxima and neutral points. 

 The latter maximum corresponds to about 



(the former being somewhat too large) , which is smaller that the values form- 

 erly found for pure Swedish iron, as was to be expected. 



A number of supplementary experiments were made to see whether the 

 observed A/ = o for currents below 0.02 ampere might not be equivalent to an 

 initial small minimum. But A/ remained persistently zero, while currents 

 decreased from 0.02 to o.ooi ampere. At 0.004 ampere the field was reversed, 

 but no significant A/ could be detected. The fringes just moved (A^ = o.i) 

 when i \vas about 0.035 ampere, indicating a field of 3 or 4 gauss. 



A rough test made of the equation by pulling the rod rr forward by the 

 backstop-screw M, figure 29, gave corroborative results. We have 



A/= 



\ 



)A<? = o. 

 J 



b 

 and if A^> refers to the turns of the screw, 



