Spring Dynamometers

Abstract

IN a former brief communication of mine on the subject of dynamometers (NATURE, vol. xiii. p. 385). suggested by an incidental remark made by Mr. Bottomley, I observed that “about three years ago Prof. Ball when introducing the C. G. S. system of units into the course of mechanics in this College had a series of dynamometers in absolute measure specially constructed for him.” In reference to this statement, Dr. Ball's successor in the chair of mechanics, Prof. Hennessy, points out, in a letter to NATURE (vol. xiii., p. 466), that “the system actually employed is not that referred to by your correspondent; I generally employ the kilogram, metre, and second, and sometimes the foot, pound, and second, to measure a dynam or unit of force.” It is, however, evident that the few words in my former letter did not question the merits of any particular system of units; whether the use of a mixed system of kilogram-metres and foot-pounds be an improvement upon a system now generally coming into use is a matter of opinion. And though the subject can hardly be one of much interest to your readers, I may, perhaps, remark that so far as my statement concerns Dr. Ball it is perfectly accurate; he was in the habit of using the C. G. S. system in his classes here, and I was unaware any change had been made in this respect, the following statement occurring in Prof Hermessy's, own syllabus for the present as well as last session:—“The unit of force employed is the ‘dyne,’ or that force which, acting uniformly upon one gramme for one second, will give it a velocity of one centimetre a second.” Even if reference had been made to Prof Hennessy, one would naturally have concluded that the printed syllabus, authorised by the Department, was the one “actually employed.”