338 Mr. R. H. M. Bosanquet on Electromagnets. 



A thick cored carbon of 13 to 15 millim. diameter is preferred, 

 as it gives a good luminous crater and burns slowly. A 10- 

 millim. copper-plated carbon is used for the upper electrode, 

 and it is adjusted so that its centre falls slightly in front of 

 the centre of the lower carbon, thereby causing the crater to 

 send its light forward. 



The lamp as used in commerce has no focusing-arrange- 

 ment. In adapting it to the Duboscq lantern, the frame was 

 made narrow; so that when the inner chimney of the Duboscq 

 lantern was removed, the lamp could be dropped entire down 

 the outer chimney, a metal sleeve of the same diameter as 

 the inner chimney being added to the lamp as a guide. At 

 the bottom of the lamp a gun-metal tube was added, tapped 

 inside with a screw-thread, into which works a steel screw 

 having a small hand- wheel near its lower end and a pointed 

 pivot at the extremity. The lamp slides down the chimney 

 of the lantern until the pivot touches the base-board. When 

 the arc burns down the lower carbon, so that the luminous 

 crater is no longer in the optical focus, a turn given by hand 

 to the wheel suffices to raise it to the proper position ; but 

 the lamp will burn for ten minutes without requiring any 

 readjustment on this account. The lamp shown to the Physical 

 Society was constructed by Mr. E. Rousseau, Instructor in 

 the Physical Workshop of the Finsbury Technical College, 

 assisted by Mr. A. D. Raine, now Demonstrator in the City 

 and Guilds Central Institution. 



XXXVIII. Electromagnets. — VII. The Law of the Elec- 

 tromagnet and the Laiv of the Dynamo. By R. H. M. 

 Bosanquet, St. Johns College, Oxford. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



THE present communication w r ill consist of two parts. First, 

 the application of the measures of the bars with pole- 

 pieces, contained in No. V. of this series (Phil. Mag. [5] xxii. 

 p. 298), to the establishment of the type of law which governs 

 electromagnets of this description, and the comparison of 

 this law with the various assumptions which have been made 

 on the subject, and in particular with Frolich's law and the 

 law of tangents. 



Secondly, a few propositions will be stated which offer a 

 general method of discussing the action of dynamos, inde- 

 pendent of the assumption of any particular law of magneti- 

 zation, and based on a consideration of the dynamic action. 



