32 



DISPLACEMENT INTERFEROMETRY BY 



TABLE 3. Elongation of an iron rod, length 43 cm.; diameter 0.67 cm. 



Factor 4-5Xio- 7 . 



= uojgauss. 



The curve in figure 33 crosses the axis (original length restored) in about 

 the same strength of field as in figure 32. The curve is quite as clearly indi- 

 cated, as may be expected (owing to the difficulties cited); but the higher 

 observations (H=2oo) are decidedly smaller contractions than are seen in 

 figure 32. The reason is to be found in the method of observation in triplets, 

 where (curiously enough) the third reading (field zero) is an apparent contrac- 

 tion in relation to the first reading in the absence of the field. The probable 

 cause of this has been suggested above. 



Figure 33 seems to indicate that the curve is approaching an asymptote, 

 or, in other words, that the susceptibility implied in equation (3) is vanishing. 



FIG. 34. 



The apparatus in the preceding experiments was believed to be faulty in 

 design, inasmuch as the clutch of the contact lever and of the coil were attached 

 to the same rigid standard. This arrangement was therefore modified, so that 

 the two mountings were quite independent, whereupon the anomalous results 

 specified largely receded. The new data did not, however, differ essentially 

 from the old and may therefore be omitted. 



As a second test a rod 28 cm. long of Swedish iron was inserted, the extra 

 length being pieced out by brass tubing soldered to each end, so that the iron 

 lay quite within the coil. The data obtained are given in figure 34, chiefly 

 in relation to higher fields and obtained with moderately large fringes. Data 



