188 Messrs. Edser and Stansfield on a Portable 



the instrument (fig. 1). Two grooves (seen in plan at C C) 

 are made along this tube, a cross head F (fig. 1) on the screw 

 E F fitting into them. This screw is drilled along its whole 

 length to admit a thick wire H I 0, the latter having a cross 

 head I, also fitting into the longitudinal grooves. These two 

 cross heads are then connected to the two ends of a spiral 

 spring, in such a manner that they are pressed by it against 

 opposite sides of the grooves. The suspending strip being 

 connected to the central wire H I at 0, its tension can be 

 increased or diminished by means of the nut K, without 

 altering the position of the coil. Any sudden jerk will also 

 be taken by the spring, thus obviating the risk of stretching 

 the suspending strip. Scratches on the wire H I C near H 

 indicate the tension used. 



As a source of current a Hellesen dry cell is used. When 

 joined up through 50 ohms the E.M.F. of the cell is practi- 

 cally constant, whilst its internal resistance is negligible*. 



The resistance of the instrument having been made up to 

 50 ohms, it follows that its sensitiveness can be varied by 

 introducing an independent resistance in the circuit. 



Let C = constant of instrument (i. e. field for 1° of torsion, 

 with no external resistance in circuit) ; 

 n = multiple of 50 ohms in circuit, exclusive of resist- 

 ance of instrument ; 

 6 = mean angular torsion ; 

 then 



Field in C.G.S. measure = C (n + 1) 0. 



C was determined, and the instrument calibrated, between 

 the coils of a galvanometer of the Gaugain type through which 

 a known current was passed. For an E.M.F. of 1*45 in the 

 dry cell it was found to be '293. The error shown in the 

 calibration was always below 2 per cent. 



By permission of the Committee of Experts, and of a num- 

 ber of the firms exhibiting, a series of measurements were 

 made at the Electrical Exhibition, Crystal Palace. The 

 results obtained are shown in the remaining figures. 



Fig. 1 (Plate IV.) shows the fields measured at various 

 distances from different dynamos, the distances plotted as 

 abscissas and the fields as ordinates. It is noticeable that 

 machines of the multipolar type show a much steeper curve 

 than other dynamos. This is especially noticeable in the case 

 of the Gulcher Dynamo curve (G). 



* See Eledroteclmische Zeitschrift, August 1, 1890, vol. ii. No. 31. 

 Bepublished in pamphlet form by Siemens Bros, and Co., Ltd. We 

 have independently verified these results. 



