C. C. Hutchins — Boys Radiomicrometer. 251 



smoked glass pressed quickly upon it. In this way the metal 

 is pressed while melted into a thin leaf. I have sometimes 

 succeeded in producing leaves of the bismuth-tin alloy as thin 

 as O02 mm . They are commonly, however, much thicker. 



A sound bit of one of these leaves, if too thick, is worked 

 down under the finger upon fine sand paper. No trouble will 

 be found in so making a plate as thin as 0*04: mm , and no partic- 

 ular care is demanded. Very narrow bars may be cut from 

 the plate by laying it upon glass, covering as narrow a portion 

 as is desired with a second bit of glass, and cutting off the pro- 

 jecting part by repeated strokes of a sharp knife or graver. 

 fey this method of proceeding the bars are rapidly produced, 

 and half an hour suffices for making a supply. The electrical 

 resistance of these alloys is so high that they are easily made 

 too thin ; 0'06 mm to 0-07 mm will be found better than thinner, 

 and bars of this thickness may have a length of 6 mm to 10 mm , 

 and a breadth of 025 mm to 0'5 mm , according to the remaining 

 dimensions of the circuit. 



The Conducting Loop. — ■ With a view to obtaining uniformity 

 of composition, extreme lightness and rigidity, the conducting 

 loop is prepared as follows: Some pieces of soft brass wire 

 No. 27 are well cleaned and silver-plated in a bath of pure 

 silver cyanide dissolved in potassium cyanide. A weak current 

 should be used to insure an even deposit, and the wires should 

 be occasionally removed from the bath and rubbed with crocus- 

 cloth. The deposit may be allowed to reach a thickness of 

 # 04 mm , when the wires are washed and dried. 



One of these wires is now bent into a loop of the form and 

 dimensions desired for the finished article; then laid flat and 

 the silver coating filed away on either side. The loop is now 

 suspended from a platinum wire in warm hydrochloric acid, 

 and the brass soon dissolving, there remain two beautiful loops 

 of pure silver, one from the outer and one from the inner parts 

 of the wire. It is evident that the peculiar sectional shape of 

 these loops gives them great lateral rigidity, so that they can 

 be made three to five times lighter than a No. 36 wire, and 

 retain their shape and permit of easy handling. 



In the same way loops may be made of copper, by plating 

 upon an aluminum wire, or of gold ; but I have found the silver 

 most satisfactory. 



Assembling the Circuit. — Having now the bismuth and anti- 

 mony bars and the conducting loops, we select a pair of the 

 former as nearly alike as possible, and solder one end of each 

 to a disc of thin copper foil l mm to 2 mm in diameter, one bar on 

 either side of the disc and near its edge. The bars should lie 

 just parallel. A thin scale of mica is pushed between them, 

 the silver loop brought into place and soldered. The soldering 



