C. C. Hutchins — Boys Radiomicrometer. 253 



of 12 sec. — about as long as can be used unless precautions be 

 taken to shield the instrument from temperature changes. 



Condensing Mirror. — For ordinary purposes the instrument 

 is vastly improved by placing behind the copper disc a small 

 silver or platinum mirror about l cm diameter and the same or 

 less focus. It is mounted upon the end of a metal plug which 

 moves with friction in a metal tube at the back, that is, oppo- 

 site the heat-receiving opening of the instrument. Heat is 

 received through a diaphragmed tube, whose diameter may be 

 the same as that of the mirror. The advantages of the con- 

 densing mirror are : First, it renders the instrument ten to 

 twenty times more sensitive by increasing the effective heat- 

 receiving surface ; and, second, the sensitiveness may be in- 

 stantly changed through wide limits by more or less withdraw- 

 ing the mirror, the highest degree of sensitiveness correspond- 

 ing to the position in which the copper disk is just at the focus 

 of the mirror. Under such conditions of temperature change, 

 convective currents, etc. as are found in an ordinary room, it 

 is not to be expected that the zero point of so sensitive an 

 instrument will remain fixed. When using a very sensitive 

 circuit with long period, although it hung in a block of iron a 

 kilo in weight, yet merely breathing in its direction from the 

 distance of a meter or more gave a deflection greater than the 

 length of the scale. I therefore surrounded the heat-receiving 

 tube with a thick jacket of wood, then swathed the whole 

 instrument in many yards of bunting, torn into strips, until the 

 whole became a huge ball. 



It is hoped that the above notes will be of assistance to those 

 who may wish to construct and use this most beautiful but very 

 troublesome instrument. 



Searles Physical Laboratory, 

 Bowdoin College, Brunswick, Maine. 



