Fig. 142.
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This micrometer head is so constructed that one complete revolution of it causes the pointer to exactly travel over the space of one division on the circle.
The head itself is divided into 10 primary parts, which indicate single minutes, and these are subdivided into 6 parts of 10 seconds each, therefore in order to measure the exact position of the pointer in Fig. 141 it is only necessary to turn the head M until the pointer is exactly over the previous division of the circle (as shown in Fig. 142) and read the distance on the micrometer head M. In this case the head has been turned through six main divisions of 1 minute = 6 minutes and two subdivisions of 10 seconds = 20 seconds, giving a total reading of 6′ 20″, this, added to the circle reading of 227° 30′, gives 227° 36′ 20″, which is the correct reading of the angle.
It will be seen that this method is very much simpler and a great deal more accurate than any form of vernier reading, and also that its greater accuracy permits the use of smaller instruments. Thus a 5-inch micrometer reading theodolite is more accurate than a 6-inch one with verniers.
Six-inch micrometer theodolites are usually divided to read to 5 seconds of arc. The method of reading is the same as described above, but in this case the circle is divided to spaces of 5 minutes each and the micrometer head to 5 main divisions of 1 minute, each of these having 12 subdivisions of 5 seconds, which it is possible to again subdivide by estimation and so measure angles to 2½ seconds.
Another feature in favour of micrometer reading instruments is the ease with which they can be adjusted. With verniers, should they get out of adjustment through damage, the instrument must be returned to a maker; with micrometers, if through rough usage or accident, it is found that after bringing the pointers to the centre of their V's and setting the micrometer heads to 0 they are not exactly opposite one another (180 degrees apart), then their setting has become disturbed and must be readjusted in the following manner:—First bring the V of one micrometer to the 360° on the circle, then see if the V of the opposite micrometer is exactly at 180°, if not this can be easily set to it by means of the small adjusting screw which will be found at the left end of the micrometer box, that is, the opposite end to the divided head. Having examined the V's and adjusted them if necessary, the next step is to set the pointers P exactly to 360° and 180° respectively, in which position the divided heads should both read 0; if they do not do so reset them as follows: Take a screw-driver and slacken the small screw which is in the centre of the divided head; this will free the divided rim so that it can be turned without shifting the position of the pointer. Turn the divided rims until they read exactly 0 at the index line and retighten the screws. This completes the adjustment.
