220 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



Paris (through Queen & Co. of Philadelphia) with the Le Chatelier 

 2)yrometer. 



The emf. was measured in microvolts (international) by the Pog- 

 gendorfF null method modified for rapid and convenient working. 

 The disposition of apparatus is shown in Figure 1. i? is a battery of 

 sufficiently steady emf. (A single Samson-Leclanche cell was en- 

 tirely satisfactory.) In direct circuit with this were two water rheo- 

 stats, W, in series ; an ammeter, A, which was a Weston voltmeter 

 (No. 395) with the calibrating coil only in use ; and a manganine 



wire resistance, a, b, c, d, 

 divided into sections, 

 each of accurately known 

 resistance. T is the 

 thermo-couple connected 

 through a sensitive gal- 

 vanometer, G, and key 

 to any desired sections 

 oC the coil o, b, c, d. 

 Tiie water rheostats were 

 of about 100 olims and 

 8 ohms respectively, and' 

 the vertical motion of 

 theirplungers thus served 

 to give a coarse and fine 

 adjustment to the re- 

 sistance in the circuit. 

 The current could thus be promptly and closely adjusted. The volt- 

 meter was one of the type having a " calibrating coil," that is, one 

 having a connection by means of which the usual high resistance series 

 coil could be cutout, leaving its resistance about 117 ohms. Any of 

 the Weston voltmeters with a special connection made to effect that 

 result would answer equally well. The voltmeter was preferred to a 

 mil-ammeter as probably more reliable. The instrument was carefully 

 and repeatedly calibrated throughout its scale by an application of the 

 Poggendorff method, measuring by the Clark cell the drop of poten- 

 tial in a known resistance through which a current was passing in 

 series with the ammeter, and at the same instant reading the ammeter. 

 The calibrations at different times checked at the same point, with an 

 average deviation of only a few hundredths of one percent. A test 

 for temperature error showed a change of but 0.1 percent for a change 

 of 15° C. ; so that, as the temperature during the work was constant 



