The same remark applies to measurements by means of the camera, where the image of the thread is projected against the reflected image of the subsidiary scale laid alongside the microscope. In this case the value of the subsidiary scale divisions must be obtained from the divisions of the stage micrometer, coinciding as nearly as possible with the position occupied by the thread. Before commencing a measurement the screens are moved about till both images appear equally bright.
Threads up to about one twenty-thousandth of an inch in diameter may be sufficiently well measured by means of a Zeiss "4 centimetre apochromatic object-glass" and an eyepiece "No. 6" with sixteen centimetre tube length. [Footnote: The objective certainly had "4 cm." marked on it, but the focal length appeared to be about I.5 mm. only.]
[§ 86. Drawing Threads by the Catapult. —]
The bow-and-arrow method fails when threads of a greater diameter than about 0.0015 inch are required — at least if any reasonable uniformity be demanded, and no radical change in the bow and arrow be carried out.
Thus in the writer's laboratory a thread of about this diameter, within 1/10000 of an inch-13 inches long and free from air bubbles — was required. A fortnight's work by a most skilful operator only resulted in the production of two lengths satisfying the conditions.
The greatest loss of time occurs in the examination of the thread by means of the microscope.
Threads for galvanometer suspensions are conveniently from 0.0001 to 0.0004 inch in diameter, and are much more easily made and got uniform than thicker threads, to the production of which the catapult method applies.
A reference to the diagram will make the construction of the instrument quite clear. The moving end of the quartz is attached to a small boxwood slider working on a tubular girder or between wires. The quartz is secured in position by clamps shown at A and B, and motion is imparted to the slider by a stretched piece of catapult elastic (C). An easy means of regulating the pull of the elastic is to hold it back by a loop of string whose length can be varied by twisting it round a pin.
Fig. 69.
