40 



A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE 



the current density was found to be of the order of 650 amp. /cm 2 . This 

 was at about 300 revolutions per minute of the disk. 



As a rough check on this, the electromotive force induced in the copper 

 was computed from the observed flux and the speed of the disk. The 

 potential gradient was found to be of the same order of magnitude as 



Fig. 10. Magnetic lines and current loops on surface of rotating disk, 

 arrow shows direction of rotation. 



Long 



that derived from the galvanometer observations above, namely, about 

 one-thousandth of a volt per centimeter. From these data we estimate 

 the total current in the disk to have been not less than 2000 amperes. 

 (IV) Effect of eddy currents on the flux through the magnet coils. 

 In order to measure the diminution in total flux when the disk was 



