﻿18 Mr. F. A. Paget on a New Form 



producing the differences in pressure required to accelerate the 

 process of nitration, and of obtaining the necessary vacuum for 

 evaporation ; it is equally adapted to purposes to which neither 

 the mercury nor the ordinary pumps are in any way appli- 

 cable. By its aid it is possible to calibrate a thermometer 

 with the greatest accuracy, and to estimate the vapour-tension 

 of such corrosive bodies as bromine, chromyl dichloride, &c. 

 by the simplest method possible, in which the necessary opera- 

 tions require scarcely more time than an ordinary determina- 

 tion of a boiling-point. 



I purpose returning to these applications of the instrument 

 in a future communication. 



II. On a New Form of Permanent Magnet. By Frederick 

 A.Paget, C.E. ; M. Soc. Civil Engineers of France ; Corr. 

 Mem. Franklin Institute ; M. late Government Commission on 

 Chaincable- and Anchor -proving Establishments*. 



^TC7ITH0UT any distinctly given reason, it is taken for 

 » ▼ granted in all works on magnetism, and in all the prac- 

 tical applications of magnetism, that it is impossible to magne- 

 tize a plate except in the direction of its greatest length. 



Michell, in his ( Treatise on Artificial Magnets/ gives a deter- 

 minate proportion, but without stating any reason, between the 

 length and the weight of magnets. A magnet, for instance, 

 2 inches long should weigh one-tenth of a pound. Cavallo re- 

 commends a width of one-tenth, Fuss one-sixth, Mussche- 

 broeck and, later, Coulomb one twenty- fourth, of the length. In 

 all these cases it is assumed that the direction of the poles must 

 be parallel with the longest dimensions of the solid bar or plate 

 to be magnetized, and that it is impossible to regularly magne- 

 tize a square plate, and still less an oblong plate, in a direction 

 transverse to its major axis. That this is correct with a solid 

 continuous plate can be easily proved by experiment ; and it is 

 well known to instrument-makers that it is impossible to per- 

 manently magnetize a square steel plate. No doubt such results 

 would greatly vary with the constitution and state of the steel 

 employed, the relations of its different dimensions, the mode 

 of magnetization adopted ; but the only experiment bearing on 

 the question that I can discover, after much research in scientific 

 works, is that of De la Bornef, who found, on magnetizing steel 

 disks, that as long as they were whole they showed no polarity, 

 and that their polarity only appeared when they were cut in two. 

 Dr. Lamont, in a paper which first appeared in PoggendorfFs 

 Annalen (vol. cxiii.), and was communicated to the Philosophical 



* Communicated by the Author. 

 t Pogg. Ann. vol. lxxii. p. 26. 



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