B. A. Unit of Resistance. 231 



coils were respectively 30-171, 9'786, 10-545, and 30.775 cms. 

 from the mean plane of the disc, itself placed as nearly as possi- 

 ble midway between the ends. The cylinder is about 66 cms. 

 long, 100 cms. in diameter, and 1cm. thick; it is thus the long- 

 est ever used in work of this kind. The flanges and cylinder 

 were cast in one piece, and the tooling was all done without 

 once removing the casting from the lathe. The walls of the 

 channels were left very thick to prevent spreading during the 

 winding of the coils. The radius of the disc was so chosen 

 that an error in its value should enter as slightly as possible in 

 the value of the coefficient of induction. 



The disc was brass, 21 - 5 cms. radius and *5 cms. thick. It was 

 fixed to a brass axle, 3 cms. diameter, turning in bearing boxes, 

 carried by suitable framework fixed inside the cylinder. There 

 was a cone of grooved pulleys toward one end of the axle, used 

 for getting different speeds of the disc. The motor for run- 

 ning the disc was in the adjoining room about 10 meters from 

 the disc. The speed obtained varied from 26 to 47 revolu- 

 tions per second, higher than has usually been used. 



The current was taken from the edge of the disc 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 disc, one brush occupying a length of 2 cms. or more on 

 the edge. The strips were made of various lengths in order to 

 avoid systematic vibrations. For the contact at the center, a 

 conical counter-boring was made in one end of the axle, and a 

 brass point was pressed into it constantly by a stiff spring. 

 The counter-boring in the axle, the point, the brushes and the 

 edge of the disc 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 meg- 

 ohms. 



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 chrono- 

 graph, 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 galvan- 

 ometer was usually very steady, showing that there were no 

 sudden changes. Every hundredth revolution of the disc 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 



