NO. 8 SAMUEL PIERPONT LANGLEV ABBOT II 



pile, but indirectly, by causing the feeble energy of the ray to modulate 

 the distribution of power from a practically unlimited source. 



" To do this I roll ' steel, platinum, or palladium into sheets of 

 from i/ioo to 1/500 of a millimetre thickness ; cut from these sheets 

 strips one millimetre wide and one centimetre long, or less ; and unite 

 these strips so that the current from a battery of one or more Daniell's 

 cells passes through them. The strips are in two systems, arranged 

 somewhat like a grating ; and the current divides, one half passing 

 through each, each being virtually one of the arms of a Wheatstone's 

 Bridge. The needle of a delicate galvanometer remains motionless 

 when the two currents are equal. But when radiant heat (energy) 

 falls on one of the systems of strips, and not on the other, the cur- 

 rent passing through the first is diminished by the increased resis- 

 tance; and, the other current remaining unaltered, the needle is 

 deflected by a force due to the battery directly, and mediately to the 

 feeble radiant heat, which, by warming the strips by so little as 

 i/ioooo of a degree Centigrade, is found to produce a measurable 

 deflection. A change in their temperature of i/iooooo degree can, I 

 believe, be thus noted ; and it is evident that from the excessive thin- 

 ness of the strips (in English measure from 1/2000 to 1/12500 inches 

 thick) they take up and part with the heat almost instantly. The 

 instrument is thus far more prompt than the thermopile ; and it is 

 also, I believe, more accurate, as under favorable circumstances the 

 probable error of a single measure with it is less than one percent. 

 When the galvanometer is adjusted to extreme instability, the prob- 

 able error of course is larger ; but I have repeated a number of 

 Melloni's measurements with the former result. 



" I call the instrument provisionally the * Bolometer,' or 'Actinic 

 Balance,' because it measures radiations and acts by the method of 

 the ' bridge ' or ' balance,' there being always two arms, usually in 

 juxtaposition, and exposed alike to every similar change of tempera- 

 ture arising from surrounding objects, air-currents, etc., so that the 

 needle is (in theory at least) only affected when radiant heat, from 

 which one balance-arm is shielded, falls on the other. 



" The first measures, on nearly homogeneous rays in the diffrac- 

 tion (reflection) spectrum, ever taken by any one that I know of, were 



* Experiments are now in progress with still thinner films of metal produced 

 by electrical or by chemical deposition. I have had the good fortune in experi- 

 ments now making in this direction, to secure the aid of Professor A. VV. 

 Wright of Yale College, and of Mr. Outerbridge of the United States Mint at 

 Philadelphia. 



