This small movement is reduced by a train of six levers, the long arm of each bearing on the short arm of the one before it. The movement of the last lever of the train is thus reduced to 1 / 4,000 of that of the screw point, so a movement of 1 / 4,000 × 1 / 25,000 × = 1 / 1,00,000,000 inch is obtained!
How can such a movement be judged? A telephone and voltaic cell are joined to the last lever of the train and to the object whose movement is under examination. If they touch, the telephone sounds. An observer listens in the telephone, and if the object moves for any reason he can find out how much it moves by turning the screw until contact is made again.
Out of the many applications of this apparatus three may be given.
(1) A short bar of iron when magnetised elongates about 1 / 1,000,000 of its length. If further magnetised it contracts. These changes can readily be measured with the instrument.
(2) The smallest sound audible in the telephone is due to a movement of the diaphragm of the telephone by about 1 / 50,000,000 of an inch. This has been actually measured by Dr. Shaw and is by far the smallest distance ever directly recorded. It is about twice the diameter of the molecules of matter.
(3) Dispensing with levers, the screw alone is used for rougher work. Dr. Shaw has shown that one hundred-thousandth of an inch is the smallest dimension visible under a microscope. By fitting an electric measuring apparatus to the microscope carriage it becomes quite easy to measure minute distances. The microscope contains a cross wire which, when the object has been laid on the microscope stage, is centred on one side of the object. The electric contact screw is then advanced till it makes contact with the stage and a sound arises in the telephone. A reading of the screw disc having been taken, the screw is drawn in and the microscope stage is traversed sufficiently to bring the wire in line with the other side of the object. Once more the operator makes electrical contact and gets a second reading, the difference between the two being the diameter of the object. In this manner the bacillus of tuberculosis has been proved to have an average diameter of 31 / 250,000 of an inch.
The same method is employed to gauge the distance between the lines on a diffraction grating.
FOOTNOTES:
[1.] April 19th, 1901.