252 C. C. 1 hit chins — Boys Radio-micrometer. 



is the only difficult part of the whole construction, and will 

 probably try the hand and patience of the beginner. A mag- 

 nifying glass is of assistance, and a very sharp-pointed tool 

 should be used. Of course, only the minutest quantity of solder 

 is permissible. A solder having a low melting point is desira- 

 ble, and I have found an alloy of tin, bismuth, cadmium and 

 lead, melting at about 80°, in every way suitable. 



The circuit being successfully assembled, the mica is re- 

 moved, and it only remains to thin the silver loop by holding 

 it for a moment in strong nitric acid and quickly transferring 

 to clean water. This process may be repeated, and the thin-, 

 ning carried beyond the point at which the surface tension will 

 hold the sides of the loop together when it is lifted from the 

 water. When thus made very thin indeed, it is well washed 

 in water and in alcohol, and completed by blackening the little 

 copper disc with Chinese ink, or drop-black. Instead of solder- 

 ing a wire to the top of the loop for attaching it to the mirror 

 staff, I have found it better to furnish the latter with a flat- 

 tened hook upon which to hang the circuit. Thus it may be 

 readily taken off for alteration, or another circuit substituted, 

 while in no way disturbing the suspension. 



The Mirror. — The mirror is made as usual by cutting a small 

 square or oblong from a silvered microscope cover-glass. It 

 was after condemning as worthless several excellent circuits 

 that I learned that the mirror was causing the trouble. The 

 quartz suspension was very fine, and after removing the circuit 

 from its hook, four complete turns of the fiber were required 

 to sensibly alter the position of the mirror alone. Not all 

 mirrors may be as magnetic as this, but when we are dealing 

 with such minute forces too great precaution cannot be taken. 

 A house of soft iron was constructed for the mirror to swing 

 in, and this being done the difficulty was removed. 



The Suspension. — Nothing remains now but to suspend the 

 circuit from a very fine quartz fiber, and provision should be 

 made in the instrument for allowing this fiber to be of any 

 length from 10 or 12 down to l cm . It is advisable to first sus- 

 pend the circuit in a glass tube or jar, away from magnets, and 

 to test its period of vibration. If this period be less than 20 sec, 

 the fiber is too coarse. Having found a suitable suspension, the 

 circuit is hung in the instrument, when, if all has gone well, 

 the period will not be greatly altered, and the circuit will be 

 completely under the control of the suspension. In case the 

 circuit is slightly magnetic the period will be less than the free 

 period ; but if diamagnetic, greater. However it may be, it is 

 easy to adjust the length of the suspension to give any period 

 desired. In one instance a fiber only 8"' U1 long gave a period 



