28 BEPOET— 1891. 



the circuit, an electric current will flow round. Eoughly speak- 

 ing, it consists of a circuit of one turn in the form of a square 

 or rectangle approximately 1 square centimetre in area ; three 

 sides of which are of fine copper wire, the upper side being 

 1cm. long, while the fourth or lower side is a compound very 

 light bar of antimony and bismuth, each piece being about 

 5 X 1 X J mm., soldered edge to edge. This rectangle is carried 

 by a thin rod on which is a small mirror, the whole being sus- 

 pended from the upper end of the rod by a very fine quartz 

 fibre. This single circuit is then suspended so that its plane 

 is parallel to the lines of force of the field of a powerful magnet. 

 A beam of light falls on the mirror and is reflected on a scale. 

 If now the least amount of heat-radiation fall on the junction 

 of the antimony and bismuth, an electric current will traverse 

 the circuit, which then tends to set itself across the lines of 

 magnetic field. The least motion of the circuit, however, causes 

 the reflected beam of light to travel along the scale, and it can 

 be thus easily seen and measured. With one of these instru- 

 ments the inventor got a deflection of the reflected beam on the 

 scale of 38mm. when the image of a candle 250 yards distant 

 formed by a mirror of 16in. aperture was allowed to fall on the 

 sensitive disc that is at the junction of the antimony and bis- 

 muth. The same candle, at the distance of about one mile, 

 would at that rate have caused a deflection of 1mm., which is 

 by no means the smallest deflection that can be accurately 

 read. 



The great delicacy of the radio-micrometer depends entirely 

 on the quartz fibre by which the circuit is suspended. To Pro- 

 fessor Boys do we also owe the discovery of the method of pre- 

 paring these fibres. Two pieces of quartz crystal are melted in 

 the oxy-hydrogen flame and drawn into small rods. One is 

 fastened to an arrowy while the other is fixed. The arrow is 

 placed in a drawn bow, both pieces of quartz are again melted 

 and joined together, and the arrow is discharged. A very fine 

 fibre of quartz is thus drawn. 



Until this discovery the most delicate suspension that \ve 

 could use for measuring very small forces by the method of 

 torsion was got by taking a single silk fibre, as spun by the silk- 

 worm, and splitting it down the middle. For any purposes of 

 measurement a silk torsion-fibre is most unsatisfactory. It is 

 viscous, and never brings the deflecting body back to the zero 

 position when the deflecting force is removed. It is also very 

 much afi^ected by moisture and change of temperature. Quartz 

 fibres have none of these defects. They can easily be drawn 

 out a hundred times finer than is the finest unspun silk. 

 They are enormously strong, stronger than the best steel, 

 being able to bear a strain of from 60 to 80 tons to the square 

 inch. 



