October 14th, 1899 letter from Nikola Tesla to George Scherff #2

Colorado Springs, Oct. 14, 1899

Dear Mr. Scherff,

I have just received clockwork, but can not tell how it will do. It was an important point to break the current just on the place I indicated in my letter. Tell Mr. Uhlman better not to push the other clockwork until further directions.

Will you tell him to prepare about 6 small lamps 3 volts as follows: A platinum wire very finely pointed and covered with glass to nearly the point, will be melted on the side of the lamp so as to barely touch the filament. The lamp with wire is illustrated in rough sketch.

The glass stem need not be very thick. I would want, say three lamps, in which the wire touches certainly the filament, and for the other three if it does not exactly touch, never mind, but it should be as close as possible. But the best way would be probably to make them all so that the wire first touches gently, then I can burn off the fine point by passing a current from the wire to the carbon. Tell Mr. Uhlman not to do it himself, as I want it done just a certain way.

I furthermore want a small Porter motor, the larger size and better make (there are two kinds), improved so arranged as to drive a small interruptor wheel fastened on its shaft. A small brush of thin platinum sheet should bear upon the wheel and Mr. Uhlman ought to try to fit as many breaks per second as he can. The current will not be very large. The best way is, perhaps, to place on the shaft (insulated from same) a wheel with teeth milled so that the brush will break the current by jumping from tooth to tooth, but I want the time of interruption long and contact very short and still a great number of breaks.

P.S. The motor should be mounted on a fibre base, and there should be four terminals, 2 on motor and 2 on break (wheel and brush). I now remember, that for some experiments I want two brushes, insulated from each other, to bear on wheel breaking the current at the same time exactly. Hoping you will be able to read this, I remain.

Sincerely,
N. T.