Take Diagram 3, in which No. 2 machine is a motor. field = 107 1 The total 10420×108 = 5.15 x 10° lines of induction. Since the area 5.15 of the diagram is 53.5 sq. cm., an ordinate of 1 cm. = × 106 = 53.5 96 x 10 lines of induction in 10°. Hence, an ordinate of 1 cm. 9.6 × 104 = 1400 lines per sq. cm. The 68.3 represents an induction of difference between ordinates at 320° and at 230° is 20; hence, the difference of induction is actually 2800. Theoretically, we have 52 In Diagram No. 4, No. 1 machine is a generator. The total field. 1 = 104×12.6 x 108 3.97 x 106 lines. The area of the diagram is 3.97 90.9 sq. cm., and therefore an ordinate of 1 cm. = x 106 437 × 10 lines in 10°. Hence, an ordinate of 1 cm. represents an induc4.37 × 101 = 639 lines per sq. cm. The difference between 68.3 ordinates at 50° and at 140° is 4-5; hence, the difference of induction 2 km C 34 × 104 × 12.9 tion of is actually 2877. Theoretically, we have 3010, as against 2877. = In Diagram No. 5, No. 2 machine is a motor. The total field 63.5 1 X × 108 4.96 x 106 lines. The area of the diagram is 104 12.3 112.2 sq. cm., and therefore an ordinate of 1 cm. = 4-42 x 10' lines in 10°. 4.42 duction of x 10 Hence, an ordinate of 1 cm. represents an in647 lines per sq. cm. The difference between ordinates at 323° and at 233° is 4.2; hence, the difference of induction is actually 2718. Theoretically, we have 2870, as against 2718 actually observed. 2 km C 34 × 104 × 12.3 = = 1.4 At page 345 of the paper on Dynamo-Electric Machinery it is shown where IF(4πnc) is the characteristic curve when C = 0, and X is the lead of the brushes. The following is an endeavour to verify this formula. The potentials both upon the magnets and upon the brushes were taken by a Siemens' voltmeter, and are rough. The speeds were taken by a Buss tachometer, and there is some uncertainty about the precise lead of the brushes, owing to the difficulty in determining the precise position of the symmetrical position between the fields, and also to the width of the contacts on the commutator. It was necessary, in order to obtain a marked effect of the armature reaction, that the magnet field should be comparatively small, that the current in the armature should be large, and the leads of the brushes should be large. The two machines had their axles coupled so that No. 1 could be run as a generator, and No. 2 as a motor. The magnets were in each case coupled parallel, and excited by a battery each through an adjustable resistance. The two armatures were coupled in series with another battery and the following observations were made : As there was uncertainty as to the precise accuracy of the measurements of potential, it appeared best to remeasure the potentials with no current through the armature with the Siemens' voltmeter placed as in the last experiment. Each machine was theretore run on open circuit with its magnets excited, and its potential was measured. the characteristic being practically straight, we infer : It has already appeared that experiment gives for I in No. 1 2.3 × 10°, and in No. 2 2·65 × 10°. The difference is probably due to error in estimating the lead of the brushes, which is difficult, owing to uncertainty in the position of the neutral line on open circuit. II. "On the Clark Cell as a Standard of Electromotive Force." By R. T. GLAZEBROOK, M.A., F.R.S., Fellow of Trinity College, and S. SKINNER, M.A., Christ's College, Demonstrator in the Cavendish Laboratory, Cambridge. Received February 17, 1892. (Abstract.) The paper consists of two parts:- In Part I an account is given of experiments on the absolute electromotive force of a Clark cell. This was determined in the manner described by Lord Rayleigh ('Phil. Trans.,' 1884) in terms of a known resistance and the electrochemical equivalent of silver. The resistance used was a strip of platinoid about 1 cm. wide and 0.05 cm. thick wound on an open frame. It was immersed in a bath of paraffin oil, and the currents used, varying from about 0.75 to rather over 1.4 ampères, did not raise its temperature sufficiently to affect the result. It had a resistance of nearly 1 B.A. unit. This was determined in terms of the original B.A. units. As part of the object of the experiments was to test the memorandum on the use of the silver voltameter recently issued by the Electrical Standards Committee of the Board of Trade, the large currents mentioned above were purposely employed. The silver voltameters were treated in accordance with the instructions in the memorandum. The standard cell to which the results are referred is one constructed by Lord Rayleigh in 1883, probably No. 4 of the cells described in his paper already quoted. The results have been reduced on the supposition that 1 B.A. unit is equal to 0.9866 ohm; if we take the number 0·9535* as representing the value in B.A. units of the resistance of a column of mercury at 0°, 1 metre long, 1 sq. mm. in section, the above is equivalent to saying that the length of the mercury column having a resistance of 1 ohm is 106.3 cm. It has also been assumed that the mass of silver deposited in one second by a current of 1 ampère is 0.001118 gramme, and that the coefficient of change of E.M.F. with temperature of a Clark's cell is 0.00076. This last result has been verified by us in Part II. * This number is the mean of the best recent results. |