﻿92 Prof. R. Threlfall on an Approximate Method 



necessary to establish the truth, or rather the approximate 

 exactness, of the theory in the simple case studied by Bosan- 

 quet before going on to apply it to other and more compli- 

 cated cases. 



Consequently I investigated the following matters : — 



(1) Influence of length of bars. 



(2) Influence of kind and size of pole-pieces. 



(3) Influence of imperfections in the ballistic method. 



(4) Cause of Bosanquet's failure to obtain agreement with 

 theory at low inductions. 



(5) Cause of similar failure (?) with an appreciable air-gap. 

 § 7. Method of Experimenting . — I wound a number of 



solenoids on brass tubes, placed the iron bars to be investi- 

 gated axially in these solenoids, observing the usual pre- 

 cautions, and measured the force (by calibrated spring- 

 balances) requisite to pull the bars apart, the force being 

 applied scrupulously parallel to the axis of the bars by means 

 ol links, pulleys, and strings. Great attention was paid to 

 the state of the cut surfaces. I tried surfaces of all kinds. 



(a) Merely filed by watchmaker's finishing files. 



(b) Ground on flat whetstones to a surface-plate. 



(c) Scraped to a surface-plate. 



(d) Ground by emery wheels. 



(e) Turned flat — to a surface-plate. This takes a little 

 skill. 



(/) Optically ground by emery and diamond-dust and 

 finished with putty-powder. 



This requires a note. Of course the bars must be provided 

 with shoes of many times their diameter to make the process 

 a success, and these shoes must be of similar material to the 

 bars. In order to save circumlocution I may state that both 

 I and my assistant Mr. Cook are fairly expert at this kind 

 of work, and we met with no real difficulty. The use of 

 diamond-dust instead of emery saves a little time, but makes 

 it more difficult to get a good result. I obtained two sets of 

 bars with properly ground faces. One of these sets was of 

 hard iron, and was not so good as Brashear's celebrated test- 

 plates, on account of a slight convexity on the part of one 

 surface and a corresponding concavity on the part of the 

 other. The other pair of bars were of soft Swedish iron well 

 annealed ; they were less than 1 centim. in diameter, and 

 about 60 centim. long. The surfaces were as good as the 

 test plates, i. e. perfect according to the present state of the 

 art. I have little doubt that they are as good soft iron sur- 

 faces as have ever been prepared. This means that there was 

 no inequality comparable with a wave-length of sodium-light 

 on either surface. 



