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XXIX. Methods of Measuring Electric Currents of great 

 Strength; together with a Comparison of the Wilde, the 

 Gramme, and the Siemens Machines. By John Trow- 

 bridge, Harvard University*. 



[Plate VIII. figs. 1 & 2.] 



THE measurement of electric currents of great strength 

 can be classed under four heads: — No. 1. The Galvano- 

 metric method ; No. 2. The Electrometer Method; No. 3. The 

 Heat Method ; No. 4. The Electrodynamometer Method. 



No. 1. The Galvanometric Method. 



With a galvanometer of small resistance and of large radius, 

 it is necessary to bring the deflection to the neighbourhood of 

 45° by means of a shunt of very small resistance. The errors 

 increase when the deflections exceed 45° in a divided circuit ; 

 and by the use of a shunt of small resistance, any error in the 

 measurement of this small resistance multiplies the whole ob- 

 servation by this error. 



By the use of a cosine galvanometer which I devised in 

 1871, and published in the ' American Journal of Science ' 

 for that year, the use of shunts can be modified ; but there are 

 difficulties, from the dip of the needle and from want of accu- 

 racy in graduations of the circle which measures the deflection 

 of the moving coil from the vertical plane. 



In practice it is very inconvenient to find a suitable shunt 

 which will answer for a wide range of experiments, and diffe- 

 rent shunts have to be used. Moreover the heating of the 

 shunt multiplies the observations by an error. In short, by 

 the use of a shunt method, we measure a large quantity by 

 observations upon a hundredth or a thousandth part of itself, 

 and proceed from a small quantity to a large one, which is a 

 fundamentally defective method. 



No. 2. The Electrometer Method. 



By means of a suitable electrometer, the difference of po- 

 tential of two points in a closed circuit can be measured ; and 

 from this the electromotive force in volts can be estimated. 

 The difficulty of dealing with static electricity in electrical 

 measurements is well known. Leakage, want of constancy of 

 charge in the electrometer, nay, impossibility of maintaining 

 a charge in certain localities, limit the use of this method, 

 even if the results obtained were not approximate. 



* From the Proceedings of the American Academy of Arts and Sciences, 

 New Series, vol. vi. pp. 122-132. Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 7. No. 42. March 1879. 



