May 31st, 1899 letter from Nikola Tesla to George Scherff

Colorado Springs, May 31, 1899

Dear Mr. Scherff,

Please tell Mr. Uhlman the following which I have carefully considered.

Suppose break is opened when 60° above temperature of room (This is a fair and safe allowance, for it will not be more). Then when hermetically closed the oil will cool down and pressure inside will be 22% smaller. Owing to this, mercury inside and oil on top - if there be any - will rise inside 6.6" above outside level. This distance we want. Therefore at least from top of oil inside of oil chamber to top of rotating shaft, after some thought I find we need 7 1/8" instead of 6.6" for entire safety. Now we will give instructions with each break to open a filling screw on top when the instrument is set in operation for the first time, and every other time when the pulley is opened. After a few minutes running the motor should be stopped and the screw tightened so as to close hermetically the pulley.

Now the central shaft and base should be cast in one piece. The top of the base will have no projecting parts so as to be able to cast it in a simple way, and the top casting made to project over the base. This is indicated in sketch.

On each side of base casting (on inside) is small rib which will be left for horizontal oil hole communicating with oil chamber.

Base just under brush looks like this.

In order to keep the oil level lower we might cut out the top casting which carries the magnets on the place where the holes are. Looked from top it will be as in sketch.

With the modifications suggested, magnet cores will be 1 1/8" longer (downwards) making core space 3 1/2" instead as in sketch 2 3/8". The cores can be made slightly thicker (take standard figure as 1/16" or possible 1/8" - do not make odd size.

Screw magnets from bottom with two long screws connecting into the cores. Also two dowel pins.

The casting carring magnets will also be screwed on base from underside.

The suggestions made in a previous sketch by Mr. Uhlman would not do, as he will see.

The top screw in cover for filling and preventing oil and mercury to come into the pulley may be made like this.

Eliminating hole for pumping in air when screw is loosened.

These modifications will make everything right.

Tell Mr. Uhlman not to write yours truly but sincerely.

Have cabled about German translation on first part of prospectus. Want it for some records in Hungary.

Yours sincerely,
N. Tesla

Please learn if Mr. Mackay and Mills are still away.
Has my friend J. J. A. called?