Melting-points of Aluminium , Silver, Gold, Copper, fyc. 39 



solidifying metal, and the other surrounded by ice. The wire 

 was that furnished by Carpentier, of Paris (through Queen & 

 Co., of Philadelphia), with the Le Chatelier pyrometer. 



The E.M.F. was measured in microvolts (international) by 

 the Poggendorff null method modified for rapid and con- 

 venient working. The disposition of apparatus is shown in 

 fig. 1. B is a battery of sufficiently steady E.M.F. (A single 



Fig. 1. 



Samson-Leclauche cell was entirely satisfactory.) In direct 

 circuit with this were two water rheostats, W, in series ; an 

 ammeter, A, which was a Weston voltmeter (No. 395) with 

 the calibrating coil only in use ; and a manganine wire resist- 

 ance, a,b, c, d, divided into sections, each of accurately known 

 resistance. T is the thermo-couple connected through a sen- 

 sitive galvanometer, G', and key to any desired sections of the 

 coil a, b, c, d. The water rheostats were of about 100 ohms 

 and 8 ohms respectively, and the vertical motion of their 

 plungers thus served to give a coarse and fine adjustment to 

 the resistance in the circuit. The current could thus be 

 promptly and closely adjusted. The voltmeter was one of 

 the type having a " calibrating coil;" that is, one having a 

 connexion by means of which the usual high resistance series 

 coil could be cut out, leaving its resistance about 117 ohms. 

 Any of the Weston voltmeters with a special connexion made 

 to effect that resi It would answer equally well. The volt- 

 meter was preferred to a mil-ammeter as probably more 

 reliable. The instrument was carefully and repeatedly cali- 

 brated throughout its scale by an application of the Poggen- 

 dorff method, measuring by the Clark cell the drop of potential 

 in a known resistance through which a current was passing in 



