Potentiometer for Thermoelectric Work, 43 



will. The tive coils to the left are permanently connected in 

 series, but are arranged so that any coil may be cut out of 

 circuit when required. Their values are 100, 40, 1, '5, and 

 •5 ohms respectively. Those to the right are a set of ten 

 simple series coils of '01 ohm each arranged so that a con- 

 nexion can be taken from any one of them to the long bus-bar 

 just in front. Suppose we wish to use as a standard a Clark 

 cell whose E.M.F. at the prevailing temperature is 1*4333 volts. 

 It is obvious that w^e may make the E.M.F. over the '1 ohm 

 balancing-coils have the desired value of 1000 microvolts 

 by putting into circuit the coils 100, 40, 1, "O, and 3 of the 

 *0l series, leading from the third hundredth to no. 17 of the 

 balancino- set, when altooether we shall have — 



in back row. 



J. in balancino-coils and brido-e-wire. 



.^ — .^.^ — ^^ ..^^^j^. 



With this connexion the maximum E.M.F. measurable is 

 18,000 microvolts. 



Should a Weston cell of E.M.F. 1*0186 be substituted for 

 the Clark, the only alteration necessary would be to short- 

 circuit coils 40, 1, and '5, and to move the connector from 

 the third to the sixth of the set of hundredths. 



The compensating current is furnished by a small 30- 

 ampere-hour secondary cell in series with which is a dial 

 resistance capable of variation from '01 up to 200 ohms by 

 steps of '01. 



The four thermojunction circuits provided are connected to a 

 selector-switch, by means of which they or the difference of 

 any two of them may be successively brought into circuit, and 

 the change over from the standard cell-connexion required in 

 the preliminary adjustment is made at the tw^o-w^ay switch 

 at the front left-hand corner, w^hichputs in the galvanometer 

 to the circuit required. 



Throughout the whole apparatus the only metals present 

 carrying current are high-conductivity copper and manganin^ 

 and all connexions on the top of the box are made by means 

 of mercury-cups. There are doubtless objections to the use 

 of mercury in any apparatus intended to be ]iermanent ; but 

 as it was essential that in this instrument the contact-resistance 

 of certain portions should be very low and constant, it was 

 thought to be, on the whole, preferable to use well-fitting 



