﻿Approximate Method of calculating Magnetic Forces, 89 



glowing in the lamp wherein the phosphorus does not burn, in 

 the lamp with carbon dioxide the filament becomes only dull 

 red, and in the others it remains obscure, but still the phos- 

 phorus burns there after a minute. 



Though the thermal conductive power interferes, I believe 

 convection to play an important part here, and I venture to 

 predict that, if the incandescent lamp, whilst glowing, could 

 be intensely cooled, by being immersed in liquid oxygen 

 or liquid air, that Prof. Dewar has so readily at hand, it 

 would at once attain a higher illuminating power*; as 

 convection would still more be lessened, the remaining 

 particles of air and mercury vapour would have their motion 

 almost entirely exhausted. And hence it may be concluded 

 that such a lamp would absorb less electric energy for emitting 

 the same amount of light, when the degree of rarefaction is 

 made the highest possible. 



The Hague, April 1894. 



VII. On an Approximate Method of finding the Forces acting 

 in Magnetic Circuits. By Richard Threlfall, M.A., 

 Professor of Physics, University of Sydney ; assisted by 

 Florence Martin, Student in the University of Sydney f. 



DURING the last three years I have had occasion to 

 design a good many reciprocating electromagnetic 

 mechanisms, and have frequently felt the want of some 

 simple method of making the necessary approximate calcu- 

 lations of magnetic forces. 



I have obtained very little satisfaction from the attempts I 

 have made to calculate tractions, proceeding by the method 

 of finding poles and applying the law of inverse squares. 



This ill success led me to investigate the applicability of 

 the methods established by Maxwell in the chapter " On 

 Energy and Stress in the Magnetic Field" ('Electricity and 

 Magnetism/ vol. ii. §§ 641-644), with the following results. 



§ 1. Theoretical Considerations. 



The problem for solution in its simplest form is as follows: — 

 " Griven an iron anchor-ring uniformly wound and inter- 

 rupted at one point by an air-gap of any given dimensions — 



* I tried this effect with a mixture of solid carbon dioxide and ether 

 on an incandescent lamp, but the globe becomes covered with frozen 

 aqueous vapour and the mixture itself is a hindrance to judging the 

 brightness acquired; a transparent cold liquid therefore is far more 

 suitable. 



t Communicated by the Author. 



