of the B.A. Unit of Resistance in Absolute Measure. 99 



adjoining room, about 10 metres from the disk. The speed 

 obtained varied from 26 to 47 revolutions per second, higher 

 than has usually been used. 



The current was taken from the edge of the disk by three 

 brushes which bore on it at angular distances of 120° ; each 

 brush was made of three or four brass strips of different lengths 

 soldered together at one end ; each strip in every brush 

 touched the disk, one brush occupying a length of 2 centim. 

 or more on the edge. The strips were made of various 

 lengths in order to avoid systematic vibrations. For the 

 contact at the centre, a conical counterboring was made in 

 one end of the axle and a brass point was pressed into it con- 

 stantly by a stiff spring. The counterboring in the axle, the 

 point, the brushes, and the edge of the disk were all carefully 

 amalgamated before each observation ; particular care was 

 given to this. The insulation resistance of the coils was found 

 to be from six to ten megohms. 



The arrangement for getting the speed differed from that 

 generally employed. As the quantity desired is the average 

 speed during the time of an observation, it seemed that a 

 chronograph, if sufficiently accurate, would give this better 

 than any other means, besides furnishing at a glance the 

 history of the systematic variations of the speed, while the 

 galvanometer showed the abrupt changes. The spot of light 

 of the galvanometer was usually very steady, showing that 

 there were no sudden changes. Every hundredth revolution 

 of the disk was recorded on the chronograph. To accomplish 

 this, one end of the axle was connected to an ordinary speed- 

 counter, consisting of a worm wheel and endless screw, 

 which rested on a board fixed to receive it. The worm 

 wheel carried a small brass pin, which made contact every 

 revolution with a brass strip fixed near it, thus closing the 

 chronograph circuit. The strip was adjustable and the con- 

 tact was always made as slight as possible consistent with 

 certainty. The duration of this contact was about -gL- sec, 

 while the clock-break was nearly twice this. 



The connexion of the axle with the endless screw was 

 made in this way: — A small hard rubber screw with square 

 head was fitted in the end of the axle and was joined to the 

 screw of the speed-counter by drawing over both a piece of 

 pure rubber tubing with thick walls, about 2 centim. long. 

 This connexion is easily made, permits no slipping, and absorbs 

 vibrations so completely that even for comparatively high 

 speeds no fastening is required to hold the counter down to 

 the board ; but for the very high speeds we used it was 

 necessary to secure it to the rubber bed on which it lay by 



