W. P. White — Switch for Thermoelement Work. 315 



bar, b, and break the previous connection before the coming con- 

 nection begins to be made. The rods then carry the frame 15 mra 

 further. Of this, 4 mm brings the frame to the auxiliary bus bars, 

 and then comes the n mm just described. The travel of the oper- 

 ating rod thus performs three operations, each of which must be 

 finished before the next begins : (1) releasing the previous con- 

 nection, thus opening the circuit ; (2) making the proper contact 

 with the auxiliary bus bars, thus putting into action the proper 

 set of potentiometer switches (or other auxiliaries) ; (3) closing 

 the circuit as it puts in the desired thermoelement, or other 

 unknown. 



The auxiliary bus bars make contact sometimes near their top, 

 and sometimes below, yet they must remain vertical. To secure 

 this a 5/16 (8 m,n ) steel rod is driven tight into the back piece, and 

 runs forward, through a very long bearing. This bearing is 

 formed by a hole in the stationary auxiliary-connecting bar, and 

 one in the front bar of the operating rod frame. The steel rod 

 passes without touching through larger holes in the bus bar cross- 

 piece, in the thermoelement bus bar support, and in the back of 

 the operating rod frame. It must of course be withdrawn (tak- 

 ing awa}^ the bus bars) before lifting off any of the structures 

 through which it passes. 



To make room for this rod, and for the rods of the movable 

 auxiliary-connecting frames, which are in line with it, above and 

 below, an interval of 40 mm is left between centers of the two 

 innermost copper connections of all the sets. 



The rod of the upper auxiliary -connecting frame slides in 

 grooves in other blocks, one of which, 20 mm high, is permanently 

 nailed upon the front of the operating rod frame ; while the 

 other, 40 mtn high, is fastened by two steel pins to the back of the 

 base so as to be removable. This block also has a hole for the 

 rod of the auxiliary bus bars. 



The indispensable eliminating switch* is not included in the 

 present description. At present we are using a triple knife 

 switch as already described,! but we have also used with success 

 something similar to a pair of operating rods, arranged so that 

 each makes the necessary three contacts, instead of two. 



For making the necessary electrical connections the various 

 copper strips are left long enough to be clamped together, thus 

 making a system without a single soldered joint, but wires can 

 also be introduced without detriment. 



Less cleaning will be needed if the contact surfaces are silver 

 plated. The parasitic thermoelectric forces developed at the con- 

 tacts are then about three times those for clean bare copper, but 

 are still ordinarily quite negligible in working to tenths of micro- 

 volts. The amount of cleaning needed by unplated copper, how- 

 ever, is less than is given to most dial switches. But if corrosive 



* Thermoelement Installations, Especially for Calorimetry. Walter P. 

 White, J. Am. Chem. Soc, xxxvi, 1859, 1914. 



f Thermoelement Installations, etc., loc. cit., p. 1859. 



