228 Intelligence and Miscellaneous Articles. 



the electromagnetic system thought it necessary to measure the 

 forces in dynes. The reason is simple : it is because, for the 

 purpose of determining the couple of the forces exerted by the 

 earth's magnetism upon a magnetized needle, those physicists have 

 employed the method of oscillations. When this method is em- 

 ployed, what is found by experiment is an acceleration ; it is then 

 natural to take the accelerations for the measurement of the forces 

 which produce them. The dyne has the advantage of greater sim- 

 plicity in the case of the oscillation-method, and in that case only. 

 Now it is very necessary to remark that the oscillation method 

 is only an indirect means ; for to measure a static couple it would 

 have been more direct to employ the bifilar suspension of Gauss, 

 or, more generally, a static method. But then no acceleration 

 occurs : one has only to weigh the magnetized needle ; the amount 

 of the force is deduced from this weight, and is obtained directly in 

 grams. It is circuitous and a complication to express it in dynes. 



The dyne system moreover complicates considerably the trans- 

 formation-formulas ; in what concerns the measurement of lengths 

 it causes the advantages of the decimal system to vanish. Elec- 

 tricians have employed as the unit of length, some the metre, others 

 the centimetre or the millimetre. Let us suppose one wishes to 

 pass from one of these units to the other. In the dyne system, or, 

 more generally, in the systems in which the force is defined by 

 means of an acceleration, the units of intensity and electromotive 

 force depend on the square root and the J power of the unit of 

 length ; so that one is obliged to divide by a factor which must be 

 calculated, and which is either the square root or the J- power of a 

 power of 10. If, on the contrary, a weight or any other force in- 

 dependent of the unit of length be taken, the unit of intensity is 

 independent of the unit of length. As to the unit of electromotive 

 force, it, like the other electrical units, is of the first degree with 

 respect to the lengths ; and the result is that we can pass from the 

 metre to the centimetre by merely transposing the decimal-point, 

 an operation which can be done mentally : this is the essential 

 advantage of the decimal system ; and we give up profiting by it 

 when we adopt the system of English units. 



In short, the electrical standards and the principal theoretic 

 formulae being independent of the choice of the unit of force, the 

 choice of that unit does not possess a very great importance, and 

 its change is always easy. The dyne presents no essential advan- 

 tage in any case. Einally, it is highly desirable that the units em- 

 ployed to measure the times, the lengths, and the forces should be 

 the same in electricity as in all other departments of Physics. This 

 unification, the aim of the excellent labours of the physicists of the 

 British Association, has not been attained, as, ever since 1863, 

 physicists have not come to an agreement to adopt the dyne. 

 Perhaps the purpose would be more easily attained by proposing 

 to electricians to take the second, the metre, and the weight of the 

 gram as fundamental units — that is to say, the now so widely spread 

 units of our metric system. — Comptes Rendus de VAcademie des 

 Sciences, Jan. 24, 1881, t. xcii. pp. 183-186. 



