A Dynamometer for Alternating Currents of Moderate Strength

Abstract

THE object of this instrument, which I had the pleasure of bringing before the Physical Society at their Oxford meeting, is chiefly medical. But it occurred to me that a few details, mainly constructive, might prove of interest. It was suggested by Mr. Preece, in consequence of a statement made by me in a paper on “Measurement in the Medical Application of Electricity,” read before the Society of Telegraph Engineers. This statement was to the effect that some difficulty still existed in the trustworthy estimation of induction currents of medium strength, such as are habitually used for physiological and therapeutical purposes. The French International Conmission had only imperfectly remedied the defiriency by recommending the universal adoption of a particular pattern of induction coil made by a single German firm, and arbitrarily graduated to a “sledge” apparatus. Mr. Preece thought that a dynamometer, which may be regarded as a galvanometer of which the moving magnet is replaced by a suspended coil introduced into the circuit, would answer the purpose; since the deflection of the coil is in one uniform direction, although the currents traversing the circuit are alternate. This very practical hint seemed to offer a prospect of obtaining accuracy in a department of science in which it is much needed. But on examining existing dynamometers I found only Weber's original instrument, which, in spite of its immense value, is fitted only for a well-appointed laboratory, and another, male by Messrs. Siemens for the measurement of very intense electric light currents, which erred on the opposite side of deficient delicacy. The dynamometer of Messrs. Siemens, shown at the French International Exhibition, by means of which the alternating currents of telephones were demonstrated, was probably in the same category, though neither I, nor the president of the Physical Society could obtain any exact details of its internal arrangements.