8 HATIO OF srEClFlC HEATS. 



tbe resiistaiice of the strip he it\. Suitahly selecting the correlative resistances 2, 

 3, 4 of tlie Wheatst<^iie l)ridge (fig. 1), the sliiliiig contact may be so uiovetl that 

 the galvanometer is without ciiri-ent, indicating the usual balance of resistances. 

 To be in correspondence with the ai>)>r«>xiinate resistance (80 ohms) of the 

 bolometer strip, the branch ^consisted of a set of standards Z (given in full in 

 fig. 5), the resistance of which conltl Ik- mkhIc very nearly ecpial to that of the 

 strip. The branches 3 and 4 containeil coils suitably wound of standard wire, 

 having about 75 ohms apiece. C)ne end of each of these was connected with the 

 bridge wire il. 



The storage cell -1, shoit-circuited thinugh a resistance box, fnmished the cur- 

 renl. From two points of the box measuring currents of suital)le magnitude could 

 thence be tapped into the bridge. Care was taken to keep down the intensity of 

 these currents, in order that appreciable healing of the bolometer strip would not 

 have to be feared. Thus the galvanometer (i when in adjustment showed no de- 

 flection, either on closing or on opening the circuit at the key n. 



AVhen the electrical or thermal condition has thus become stationarv and the 

 stopcock {?8 "f tlie receiver is then suddenly opened, the gas will [)recipitately 

 expand, cording off both itself and the Ixdometer, and the galvanometer now shows 

 a definite deflection. It is our problem to find the tem]ierature 7o of the strip at the 

 instant when the temperatwe depression of the gas ix a md.rimii/n, by availing our- 

 selves of the electrical registry Wg. Let us assume in the first place that the tem- 

 perature of the strip and that of the surrounding ga.s are at all times identical. 

 Instead of consideiing the initial pressure pi given, to find the resistance decre- 

 ment ic\-w.j corresponding to the pressure decrement y^,-/>..,, let us preferably adopt 

 an invei-se method in the following way : Start with a given resistance decrement 

 u\-u'.j; then l)y trial and erroi- continually change the initial pressure/*, until the 

 resistance tc^ of the bokmieter strip exactly coi-responds to the nniximum tempera- 

 ture depression of the gas. No doubt this is a somewhat cundjersome method, but 

 it has the advantage that the galvanometer is used with the needle in the zero po- 

 sition oidy. It is at least practicalilc in the definite and liiial measurements, after 

 the ap[iroxiiiiate values of the correlated ipiantities ^^j-^^o ^"'1 «;,-/f'o are already 

 known. 



The apjiroximate values referred to can be found either by computing back- 

 waids iVoiii know n v;dues of «, or l>y the following direct procedure. At the gas 

 pressure yy, and temperature 7\ let the bridge be adjusteil. Then let the resistance 

 of the standiirds Z be reduced from IT, to any suitable value Ho. The bi'idge is 

 thereby thrown out of atljustment and the galvanometer needle will show a deflec- 

 tion. For convenience in designation let the direction of this defiection be called 



