C. Barns — Simple Screw Micrometer. 



267 



Art. XXI. — On a Simple Screw Micrometer ; by C. Bartts. 



In my last paper* I referred to a simple type of micrometer 

 for interferometer work, consisting only of a screw with a 

 light adjustable mirror at its end. Results with the screw 

 were at first only moderately successful, so long as a single 

 socket was used. But by replacing the single support by two 

 supports a and b (fig. 1), slotting each parallel to the axis of the 

 screw SS (as shown at ss) not quite through, so that the set 

 screws on one side (elastic connection on the other) gave the 

 play the necessary ease of motion to the exclusion of all wob- 

 ble, the results were very much improved. Success was even- 

 tually reached by making the lugs or socket ab of indurated 

 fiber instead of metal, and using a carefully cut brass screw. 

 Fig. 1 shows the screw SS mounted on three leveling screws in 



B and locked at c to the table A. N is the mirror attach- 

 ment, screwed to the end of SS. 



Screws so cut (each nut one-quarter to one-half inch thick) 

 are usually too tight at first, but they cling admirably. The 

 necessary ease of motion was secured by the set screws at a 

 and b (fig. 1), which here slightly push the socket apart. The 

 ease with which a brass screw ten inches long and a pair of 

 sockets of the kind in question may be cut and mounted, and 

 their admirable precision of motion when tested by the inter- 

 ferometer, is astonishing. 



Fig. 2 shows a comparison of the new screw No. 5 with the 

 Fraunhofer micrometer, using about three inches of the former. 

 The adjustment of the mirror of the latter (No. 5) was left incom- 

 plete, so that the directly reflected ray described a small circle 

 in the telescope. The coincidence of images of the two inter- 

 ferometer mirrors M and N was secured at the beginning of 

 each of the turns, by the initial adjustment, so that the ellipses 

 * This Journal, xxxiv, p. 333-7, 1912. 



