EXPLORING THE ATOM 



The instrument with which this feat of measuring 

 infinitesimal gradations of temperature is accom- 

 plished is known as a bolometer, and was invented 

 by the late Professor Langley of the Smithsonian 

 Institution. 



The principle on which the bolometer is construct- 

 ed is that any change of temperature in a metal 

 changes the capacity of that metal as a conductor 

 of electricity. By using an excessively tenuous flat- 

 tened thread of platinum for his conductor, and an 

 exquisitely sensitive galvanometer to register the 

 effects, Langley produced an instrument which will 

 respond to changes of temperature so slight in degree 

 that no one could reasonably have supposed them 

 measurable. Indeed the feats accomplished by the 

 little instrument are as incredible, not to say fantastic, 

 as the feats of the spectroscope itself. 



A generation ago instruments for physical research 

 had attained a high stage of development; but to 

 measure a change of temperature of one-thousandth 

 of a degree was considered a remarkable feat. The 

 layman will be disposed to admit that it is a remark- 

 able feat. But the perfected Langley bolometer 

 measures a change of one-hundred-millionth of a de- 

 gree. It is competent to deal with the infinitesimal 

 quantities of heat that come to us from such bodies 

 as the moon and the brighter stars. 



As a practical apparatus, the bolometer's chief use 

 has been to test the precise quantities of heat that 

 come to us from the sun. Langley himself used it 

 for this purpose, and since his death Professor C. G. 

 Abbott has conducted an elaborate series of experi- 



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