348 PROCEEDINGS OP THE AMERICAN ACADEMY 



current ; and if there were no limit to the current which could be 

 advantageously used, other than that of the inconvenience of a large 

 battery, this method might perhaps be practically advisable, as it cer- 

 tainly is the simplest. But the heating of the thin metallic strips by 

 the current itself, imposes the necessity of keeping the latter below a 

 certain maximum value. Employing always this greatest allowable 

 current, a greater effect * will be produced with the ten narrow strips 

 tiian with the one broad one (using a galvanometer of much greater 

 resistance than that used with the single broad strip). This sub- 

 division of the metal has greatly increased the mechanical difficulties 

 of construction, and I have felt that to make the apparatus generally 

 useful I must learn how to overcome these difficulties, so that it can 

 be produced at a not too great cost for ordinary use by the scientific 

 student. It would at any time also, I repeat, have been easy to make 

 a far more sensitive iustrvunent than I am about to describe ; but, from 

 the first, my cliief aim has been to produce one trustworthy, in the 

 sense that it gives exact quantitative results. 



After nearly a year's labor (I began these researches systematically 

 in December, 1879), I have procured a trustworthy instrument. It 

 aims, as will have been inferred from the preceding remarks, to use the 

 radiant energy, not to develop force directly as in the case of the 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 t steel, platinum, or palladium into sheets of from 

 Too to 6 00 <^f ^ 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 current passing through the first is 



* We cannot say exactly how much greater, since our formulae do not take 

 account of the temperature as actually modified by re-radiation and conduction, 

 but only of the amount of lieat imparted. 



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

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

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

 of Yale College, and of Mr. Oulerbridgo of the United States Mint at riiiladel- 

 phia. 



