5. Laboratory Notes. Measurements of the Electromotive Force of the Gramme Machine at Different Speeds

1880 Proceedings of the Royal Society of Edinburgh  
The following measurements were made by means of a Gramme machine, recently procured for the University. I desired to make use of it, not only for electric light, but for electrolysis, the exciting of electromagnets, and various other purposes for which we have hitherto used from 4 to 40 or so Bunsen cells. I therefore arranged the driving-gear so that with the same motor (a 3½ h.p. gas-engine) it was easy to use either of three speeds. These are, approximately, 800, 533, and 320 turns per
more » ... 320 turns per minute. The electromotive force varies, of course, not only with the speed but with the resistance of the whole circuit—falling off at first rapidly and then more slowly for any one speed, as the resistance is increased. As I had no means of measuring the speeddirectly, I was somewhat puzzled at first to find the electromotive force at any one speed rise to a maximum, and then rapidly fall off as the whole resistance was gradually diminished.
doi:10.1017/s0370164600043443 fatcat:qy4smjq37vht5fdib5cqbpezjm