for Thermocouple Measurements. 671 



only the ebonite studs project, for the better avoidance of 

 thermoelectric effects. 



To calibrate the instrument the procedure is as follows : — 



(i.) The 10O, 100O, and 10,000 resistances are compared 

 directly with standards. To allow of this and similar opera- 

 tions, the heads of all the studs in the instrument carry 

 screws. The comparison was kindly made for me at the 

 National Physical Laboratory. 



(ii.) Each tenth-ohm is compared with the succeeding one 

 by the usual method for comparing nearly equal resistances ; 

 the slide-wire is compared with the first tenth-ohm in the 

 same way. During these operations current is led through 

 the coils and the voltage taken off at the same points as when 

 the potentiometer is ordinarily in use. 



(iii.) Groups often tenth-ohm coils in series are compared 

 with a standard ohm by the method described in the follow- 

 ing paper. 



(iv.) Further, as a check on the results, two groups of 

 ten tenth-ohm coils were measured at the National Physical 

 Laboratory in the usual way. These measurements are of no 

 direct use, how T ever, as in making them current is led into and 

 out of the coils at the points ordinarily used for taking off the 

 voltage ; hence the result differs from that obtained in (iii.) 

 by the resistance of the studs, which is one or two ten- 

 thousandths of an ohm. 



(v.) The slide- wire is calibrated. 



It would be a convenience in calibrating to provide an 

 additional terminal, connected to the point of junction of the 

 last tenth-ohm with the hundred-ohm coil, as this would 

 enable one to lead in current under the usual working con- 

 ditions without passing the high-resistance coil : this is 

 desirable when calibrating the tenth-ohms, as it is safe to use 

 pretty large currents through them. Otherwise I have found 

 the working of the instrument satisfactory. 



Before measuring a thermocouple, two tests should be 

 made. First, the galvanometer-key should be pressed half- 

 way down, so that the galvanometer-circuit is broken. The 

 needle will probably swing a little. If it swings equally on 

 each side of its previous position of rest, then 4 is no thermo- 

 electric effect in the galvanometer. Second, a short piece 

 of copper wire should be put across the thermocouple ter- 

 minals and the battery-circuit broken: if then (with double- 

 pole switch set to the thermocouple circuit) on pressing the 

 galvanometer-key there is no deflexion, this shows absence of 

 disturbing thermoelectric effects in the rest of the apparatus. 

 The measurement may then be proceeded with. 



