In 1860 Dr Antonio Pacinotti invented what is now called the toothed ring winding for armatures and described it in an Italian journal, but it attracted little notice until reinvented in 1870 by Gramme.
To start the machine the balls were brought in contact, one of the paper armatures electrified, say, with positive electricity, and the disk set in motion.
§ 68 c), consists of a wheel carrying several soft-iron armatures fixed at equal distances Rayleigh's round its circumference.
If the wheel be driven at such rate that the armatures move one place on in about the period of the current, then on putting on the current the electromagnet controls the rate of the wheel so that the agreement of period is exact, and the wheel settles down to move so that the electric driving forces just supply the work taken out of the wheel.
In an experiment described by Rayleigh such a wheel provided with four armatures was used to determine the exact frequency of a driving fork known to have a frequency near 32.