OP ARTS AND SCIENCES. 



347 



C. Delicate linear thermopile (7 couples). Working face about 

 1 mm. by 10 mm. = 10 stj. mm. Internal resistance 0.72 ohm. 



S. The iron strip, which was about 7 mm. by .176 mm. and whose 

 working face was therefore about 1 sq. mm. Internal resistance 

 0.9 ohm. 



The time of exposure was about five seconds for the thermopiles, 

 and about one half this for the strip, the latter time corresponding to 

 the rapid swing of the (designedly) insensitive galvanometer. 



In the table, the first column gives the name of instrument; the 

 second, the cross-section of the beam of radiant heat which is received 

 upon it ; the third is the actual deflection in galvanometer divisions ; 

 and the fourth the deflection for each square millim.'tre of exposed 

 surface. The fourth column then (since the radiant energy falling ou 

 the broad pile could obviously be concentrated on the narrow strip 

 with but little loss) gives approximately the relative sensitiveness of 

 these instruments, using the same galvanometer (i? = .5 ohm) without 

 considering the element of time: if that be considered, the relative 

 sensitiveness of the strip will be still greater. 



The total current employed with the last was about .58 Weber, of 

 which one half passed through the strip, and this is somewhat greater 

 than can be advantageously used in actual work. To increase the 

 efficiency of the strip, another method is used in practice, which will 

 be described immediately ; but these numbers give only a rough com- 

 parison of the efficiency of the two instruments under similar circum- 

 stances. 



Instead of a narrow metal strip, we might use one as broad as it is 

 long, or, if we desired to increase the resistance while exposing the 

 same area, we could cut this, for example, into ten narrow strips stand- 

 ing side by side, but only joined at their alternate ends, the electric 

 current being passed through the members of the series successively. 

 The single square strip, possessing -^J^ of the resistance of ten narrow 

 ones taken in sequence, could be used with a low-resistance galvanom- 

 eter and a battery, arranged for " quantity," transmitting a much larger 



