200 RICHARD THRELFALL. 
and tractive force were made. In both cases Maxwell’s theory 
2 
leads to the expression i — 
for the force in air between two opposing plane faces of iron— 
infinitesimally separated—A being the area of one of the equal 
faces ; and B the (uniform) induction density. 
The net result of Bosanquet’s work was to show : 
(i.) When Bis below 5,000 the tractions observed are gener- 
ally much too large. 
(ii.) The formula does not hold when the air gap is appreciable. 
(iii.) It holds within about 5'/ up to very high inductions, 
(18,000). 
It is obvious therefore that there is room for more work on the 
subject. 
6. For the reasons given I felt very strongly that it was necess- 
ary to establish the truth or rather the approximate exactness of 
the theory in the simple case studied by Bosanquet before going 
on to apply it to other and more complicated cases. Consequently 
I investigated the following matters : 
(i.) Influence of length of bars. 
(ii.) Influence of kind and size of pole pieces. 
(ii1.) Influence of imperfections in the ballistic method. 
(iv.) Cause of Bosanquet’s failure to obtain agreement with 
theory at low inductions. 
(v.) 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 investigated axially in 
these solenoids—observing the usual precautions ; and measured 
the force (by calibrated spring balances), requisite to pull the bars 
apart—the force being applied scrupulously parallel to the axes of 
the bars by means of 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 watchmakers’ finishing files. 
(6.) Ground on flat whetstones to a surface plate. 
