308 W. P. White — Switch for Thermoelement Work. 



ances, voltages, or currents, to permit the different measure- 

 ments to be alternated with a rapidity that greatly increases 

 precision and also to allow necessary or desirable auxiliary 

 measurements to be made in many cases. 



For many of these purposes a double potentiometer, or some- 

 thing equivalent, is almost necessary. Such potentiometers are 

 now readily obtainable and at a low cost ;* the exchanging 

 switch here described may therefore be said to complete the 

 provision of a satisfactory outfit for all such kinds of work.f 



In determinations such as those just mentioned, it is of 

 course often possible to get along with but one electrical 

 measuring instrument, and in many cases a procedure which 

 permits this lias been developed, so that the use of more than 

 one seems strange, if not over complicated. This situation, 

 however, may be regarded as the legacy from a regime of 

 crude or restricted apparatus, for as a rule the procedure with 

 only one electrical measuring instrument is obviously and 

 seriously disadvantageous.^: 



In mere convenience also, aside from the value of the 

 results, a switch like the present will in a few years more than 

 pay for its installation. 



Of course, all these considerations gain in weight as the con- 

 struction of the switch becomes easier, and it is partly on 

 account of its ease of construction that the present switch has 

 seemed to deserve a separate description. The mechanical 

 parts of our own took about 13 hours to make, including 

 experiments and mistakes ; this corresponds to a cost of about 

 $10.00 for the whole switch, since the cost of materials is prac- 

 tically nil.§ 



* Potentiometers for Thermoelectric Measurements, Especially in Calor- 

 imetry, Walter P. White, J. Am. Chem. Soc, xxxvi, 1874-5, 1914. 



f The arrangement of knife switches shown on page 1878 of the same 

 paper has been used, modified for 14 thermoelements, with complete satis- 

 faction for about a year, but it took longer to install, is not so simple in 

 operation, and is harder to overhaul than the one described in the present 

 paper. If the mechanical arrangements of the switch here described had 

 been thought of a little earlier, it would have been used instead. For 

 work with a single potentiometer and a few thermoelements (say, 4 or less) 

 the knife-switch type is probably preferable. 



\ For instance, Hiittner and Tammann describe a calorimetric method 

 wherein, in order to avoid a second thermometer in the furnace, they make, 

 regarding the furnace temperature, an assumption palpably contrary to 

 fact, as Plato has shown (Zs. anorg. Chem. xlv, 721, 190b). And Plato, in 

 his turn, though he did use a second thermometer for his own admirable 

 work, nevertheless, in trying to formulate a method which demands but one, 

 was compelled to make another assumption, also erroneous, regarding furnace 

 temperature (namely, that there is no difference between the temperature of 

 a cooling furnace and that of a charge within it.) In several calorimetric 

 methods, also, precision and convenience have often been sacrificed by the 

 lack of suitable provision for determining the temperature difference 

 between jacket and calorimeter. 



§ Of course this estimate might not apply in the case of a factory-made 

 switch. 



