. Carnegie Institution of Washington publication. t. (III) Intensity of the eddy currents.—The galvanometer leads weretouched to the disk, as described above, at a point near the magnet pole,but oriented in such a way as to produce a maximum deflection. Fromthe distance between the points of contact and the resistance and sen-sitiveness of the galvanometer, the potential difference between thepoints was found, and from this and the specific resistance of copper 1 At the time of these tests the magnetic poles were pushed in about 2 cm. from theouter edge of the disk. This can hardly have produc
. Carnegie Institution of Washington publication. t. (III) Intensity of the eddy currents.—The galvanometer leads weretouched to the disk, as described above, at a point near the magnet pole,but oriented in such a way as to produce a maximum deflection. Fromthe distance between the points of contact and the resistance and sen-sitiveness of the galvanometer, the potential difference between thepoints was found, and from this and the specific resistance of copper 1 At the time of these tests the magnetic poles were pushed in about 2 cm. from theouter edge of the disk. This can hardly have produced an appreciable change in anyof the quantities observed (c/. fig. 13). 40 A BICYCLE ERGOMETER WITH AN ELECTRIC BRAKE the current density was found to be of the order of 650 Thiswas at about 300 revolutions per minute of the disk. As a rough check on this, the electromotive force induced in the copperwas computed from the observed flux and the speed of the disk. Thepotential 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, aboutone-thousandth of a volt per centimeter. From these data we estimatethe 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 MAGNETIC REACTIONS 41 running, a single turn of wire was wrapped around one of the magnet coilsand connected to a ballistic galvanometer. The throw was measuredwhen the field current was turned on, and again when the disk was sud-denly set in rotation. The latter throw was always in the opposite direc-tion to the former; its measured value was certainly somewhat too small,since it took an appreciable time for the disk to attain full speed. Theresults indicated a diminution of the total flux
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