372 



Dr. P. E. Shaw. The Magnetic 



[May 22, 



field throughout its length. In the present research it is immaterial if 

 there be a small error in the calculated value of field; but it is 

 essential that any expansion produced should be due to the specimen 

 exclusively. 



As regards the rod, we are not troubled by the demagnetising factor 

 (N) in the well-known expression for effective field,* 



H' = H-NI, 



where I = magnetic intensity and H = 4iray. In the present instance, 

 since k is very small for all the metals in question, I becomes insensible. 



Fio. 1. 



The electric micrometer was calibrated by using a simple electric 

 micrometer! in the following way, which is accurate enough for present 

 purposes. Suppose / is the last lever of the system of levers and p 

 is the measuring point ; we require to know how many divisions of the 

 electric micrometer screw correspond to a known movement of p. 

 Remove the coil and accessories which are above /?, and bring screw s 

 of the simple micrometer down to p. When electric contact is com- 

 pleted take a reading of the graduated head of the screw s. Rotate s 

 so that the surfaces at p are separated by a known amount, say 

 4 microns. Then find out how much the screw of the electric 

 micrometer has to be rotated to bring the surfaces together again and 

 produce electric contact once more. 



As a means of four results, 4 microns movement at p corresponds to 

 14,900 divisions on the electric micrometer screw, hence 



1 micrometer division corresponds to 2'68 x 10~ 8 cm., which (since the 

 rod is 19 cm. long) is equivalent to l*4x 10~ 9 of the length of 

 the rod. 



Allowing for uncertainties in readings, I should say that three 

 divisions on the scale represent the smallest difference that can be 

 detected with certainty. 



3. As seen above, the range of field was considerable and the upper 

 limit high (1900 c.g.s.), whilst the measuring apparatus was very 

 delicate. 



* Ewing's ( Magnetic Induction,' 2nd edition, p. 24. 

 f Pliya. Kev.,' March, 1903. 



