ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 429 



circnit was made tlirougla two platinnm- faced gun-metal wheels, each 

 about 15 cm. diameter, which revolved in contact with the ivory cylinder. 

 The wheels revolved in insulated bearings about vertical axes, nearly in 

 the same plane as the axis of rotation of the coil. The tipper end of the 

 axle of each wheel carried a small copper mercury-cup into which a well- 

 amalgamated copper wire dipped for connecting the coil with the end of 

 the wire R (see figure), of which the resistance was to be measured. This 

 arrangement was adopted in order to avoid the heating, and consequent 

 thermo-electric action, which w^ould probably have resulted from the use 

 of rubbing contacts. It was found very efficient for this purpose. 



In order to avoid as far as possible the effects of self-induction in 

 the revolving coil, the platinum contact-pieces had an angular breadth of 

 only about 20 degrees, so that the coil was in metallic connection with the 

 rest of the circuit during only about ^th of each revolution. By adjusting 

 the contact-wheels so that the vertical plane containing their axes coincided 

 with the magnetic meridian, the middle of the period of contact was made 

 to coincide with the instant of maximum intensity and minimum rate of 

 variation of the electromotive force in the coil. The arc of contact 

 actually employed was 20° 3', which gives for the ratio of the maximum 

 and minimum electromotive force due to the earth's magnetism the value 

 1 : 0'98l7, or an extreme variation of less than 2 per cent. 



Putting together the values of the constant factors in the expression 

 for the resistance to be determined, we get 



^ 9-55 61 X 10« 

 '' T tan ' 



leaving T, the period of rotation of the coil, and d, the deflection of the 

 tangent-galvanometer, to be observed in each experiment. 



To determine the speed of the coil, the following method was adopted. 

 Three glass pens, each controlled by a small electro-magnet, were caused 

 to mark side by side upon a strip of paper drawn forward by clockwork, 

 as in an ordinary Morse receiver. The pens, when left to themselves, 

 ruled parallel straight lines on the paper, but when any of the electro- 

 magnets was excited, the corresponding pen was pulled to one side and 

 a notch was made in the line the pen was drawing. By means of a 

 wheel of 100 teeth, cari-ying a pin which made contact with a light 

 spring once in every revolution, and gearing into a screw cut upon 

 the upper part of the axle of the coil, the circuit of one of the 

 electro-magnets was completed for an instant at every hundredth 

 revolution of the coil, and an indentation was made in the corresponding- 

 line. The circuit of the second electro-magnet was broken for an instant 

 by a clock at intervals of one second, thus making notches on the second 

 line. By afterwards measuring the distances between the notches on 

 the two Hues, the speed with which the coil was spinning at any 

 instant could be ascertained. This measui-ement was made by laying 

 over the paper a strip of glass divided on its lower surface into centi- 

 metres and millimetres. The degree of accuracy attainable in this way, 

 independently of error of the clock, was about one pai-t in one thousand. 

 The speeds used in the experiments varied from about nine to about 

 twelve revolutions per second. The electro-magnet acting on the third 

 pen was under the control of an observer who watched the zero-galva- 

 nometer (g in the figure,) and held down a contact-key, which completed 

 the corresponding circuit whenever and as long as this galvanometer 



