'2^0 (\ (\ Hutchins — Boys Radiomicrometer. 



would be mere accident or good fortune that one would secure 

 a sample of wire yielding a circuit having a stable zero point 

 and a period of, say, ten seconds. 



Paschen* used silver wire, which he found diamagnetic, and 

 corrected this in part by drawing it through steel plates. He 

 obtained considerably greater sensitiveness than was obtained 

 by Boys. 



Whatever method of construction of the circuit be adopted, 

 the fact that no available material is magnetically neutral, 

 entirely upsets in practice any theory of the instrument that 

 may be formed. For instance, the sensitiveness of the instru- 

 ment should increase proportionally to the increase of the 

 magnetic field in which the circuit hangs ; but, inasmuch as 

 the magnetic disturbance varies as the square of the field 

 strength, diminishing the field strength may increase the sensi- 

 tiveness ; and this follows whether the material of the loop 

 be either dia- or para-magnetic. In one instance, placing an 

 iron rod across the poles of the field magnet increased the sensi- 

 tiveness of a certain circuit eight or ten times. Lewisf found 

 that a cast-iron magnet might be too strong. Too much care 

 can not be taken to employ a field as weak as other circum- 

 stances will allow, the magnetic disturbances diminishing so 

 rapidly with the field strength. These circumstances are : 

 slight sensitiveness, for a neutral circuit, in a weak field, and 

 a limit beyond which the weight of the circuit can not be 

 reduced. For supposing a circuit magnetically neutral, if we 

 employ a weak field we must employ a suspension of little 

 torsion, and meet the increased period of vibration consequent 

 upon the weakening of the suspension, by diminishing the 

 weight of the circuit. This last has the advantage of remov- 

 ing a portion of the disturbing element, so that, in the end, the 

 stability and sensitiveness of the circuit will depend upon how 

 light it can be made, as well as upon the character of the 

 material that enters into it. 



I purpose now to offer a construction which, in my hands, 

 has never failed to produce satisfactory results, yielding very 

 light circuits of great sensitiveness and stable zero point. 



Preparation of Bars for the ThermalJunction. — Two alloys 

 are prepared, J first, bismuth containing 2 per cent of anti- 

 mony; and the second, bismuth containing 10 per cent tin. 

 Usually much trouble is experienced in working these brittle 

 metals into fine bars. It is easily accomplished as follows : A 

 quantity of the metal is melted, made very hot, a small pool 

 poured upon a sheet of smoked glass, and a second piece of 



* Wied. Ann., xlviii, 272 (1893). f Loc. cit. 



%C. C. Hutchins, Thermo-electric heights of bismuth and antimony alloys, 

 this Journal, xlviii, 226 (1894). 



