710 Prof. Stroud and Mr. Gates on the Application of 



we are describing ever comes into use in physical laboratories 

 its behaviour may be better understood. In describing these 

 experiments we shall speak of a frame which sets itself so as 

 to include the maximum or minimum number of lines of 

 forces as being equator ially or axinlly stable respectively. 



Experiment 1. — A thin copper wire (No. 24) with its ends 

 joined so as to form a rectangle was axially very stable as we 

 expected. The equatorial position was also one of stability, 

 though to a much less extent. 



Experiment 2. — A similar wire with the ends insulated 

 from one another was equatorinlly stable to a small extent, 

 axially unstable. 



Experiments 3 & 4. — Precisely similar results were ob- 

 tained with a thicker copper wdre (No. 16) bent into a similar 

 shape. 



Experiment 5. — A rectangular frame was made out of a 

 strip of thin sheet copper 16 cms. long and 2 cms. wide with 

 the ends soldered together. This proved to be axially very 

 unstable, equatorially very stable. 



Experiment 6.— ^This frame behaved in just the same 

 manner when the ends were insulated from one another. 



Experiment 7. — A D^Arsonval coil with the ends joined 

 showed great axial stability (as of course was expected). 



The explanation of the behaviour of the frames in these 

 experiments is very simple. As is well knowm a coil of wire, 

 with the ends joined, in an alternating field sets itself so that 

 there is minimum magnetic induction through it, i. e. axially 

 (Expt. 7). Continuous metal sheets in which the induced 

 currents can freely circulate are vigorously repelled (Expts. 

 5 & 6), i. e. set equatorially. Thus it wall be seen that the 

 one effect or the other may predominate according to the 

 shape of the frame. 



When w-e originally devised the method here described, 

 as it happened w^e had in our possession a D'Arsonval 

 galvanometer with a field produced by an electromagnet. 

 It was easy to try the method with this instrument (after- 

 wards referred to as A) by simply substituting alternating 

 for continuous currents in the coils surrounding the field- 

 magnets. We were very well pleased with the results, 

 especially considering the very feeble magnetization of the 

 solid soft-iron core. We next had an instrument made with 

 laminated iron cores. The electromagnet was built up of 

 soft charcoal-iron stampings, 0*3 mm. thick, in the shape of 

 a hollow rectangle — one short side being missing — 21*6 cms. 

 long by 8*9 cms. wide. The limbs were 2*5 cms. broad, and 

 the stampings were first varnished and then piled up to a 



