W. P. White — Switch for Thermoelement Work. 311 



giving a very easy fit for the operating rods. It is held in place 

 bv slipping over two vertical pins in the base, and can be lifted 

 off at any time. It carries the locking bar, which is a piece of 

 strap iron 2 mm thick and 12 mm wide, bent as shown and pivoted 

 by two steel pins, P 3 , clamped to the top of the frame. In fig. 2a, 

 this bar is shown holding the second operating rod forward, by 

 means of its brass sleeve, while the rod in its turn holds forward 

 the lower auxiliary-connecting frame X 2 . The back of the oper- 

 ating rod frame is fastened to the sides with screws, since it 

 must be removed in order to take out any of the rods. 



The thermoelement leads are copper strips 0'l mm thick and 6 mm 

 in width, which enter through a wide crack left below the back 

 of the base, are fastened (with celluloid insulation, of course) to 

 the wooden strip, E (fijjs. 2b and 3) and then rise vertically, sup- 

 ported by a celluloid strip 8 cm high, over whose top their extreme 

 ends are bent as a fastening. The celluloid is cut down from the 

 top to the bar, so as to form a series of tongues, 24 mm broad, 

 each carrying a pair of leads. These leads are separated 6 mm , or 

 12 mm between centers. They run up on the back, or farther side, 

 of the celluloid.* 



To make contact, when the switch is in use, a celluloid tongue 

 is bent back by one of the operating rods, and so presses the 

 copper leads against the thermoelement bus bars. This is a 

 wooden support 20 mm square, carrying on top a copper strip with 

 teeth 6 mm wide, and 24 mm apart between centers, projecting down 

 over the first (or near) face, and another strip below with alter- 

 nating teeth projecting upward. The teeth of course stand 

 opposite the thermoelement leads. From these two toothed 

 strips the electrical connections run to the eliminating switch (not 

 a part of this switch) and thence to galvanometer, potentiometer, 

 etc., in part through the auxiliary bus bars soon to be described. 

 The two pieces of copper are insulated and held in place by sheet 

 celluloid, as already explained. The operating rods do not 

 directly touch the vertical celluloid tongues but press against the 

 obtuse apexes of the broad flat wooden wedges, W, shown in 

 figs. 2, 3. These wedges are l cm thick, 2 cm wide and about 3 - 5 cm 



* One form of leakage shielding, hitherto not described, but sometimes 

 desirable, is relatively easy with this arrangement of terminals. (For gen- 

 eral principles and methods of leakage shielding, see : Leakage Prevention 

 by Shielding, Especially in Potentiometer Systems, Walter P. White, J. Am. 

 Chem. Soc, xxxvi, 2011, 1914). If one thermoelement is in a furnace, or any- 

 where where shielding is incomplete, and is to remain there while readings 

 of maximum precision are made on another thermoelement, a troublesome 

 leakage current may enter the measuring system by leaking from the dis- 

 connected end of the furnace thermoelement at the switch, since insulation 

 alone cannot, as a rale, be relied upon to protect a sensitive thermoelectric 

 system from leakage from power circuits. The remedy is to arrange so that 

 all the pairs of thermoelement leads, when disconnected, are separated from 

 each other within the switch by a shield. To this end, each pair has a sepa- 

 rate celluloid strip, and these strips are separated from the wooden parts by 

 two metal plates which are connected to the external shield, are carefully 

 insulated from the rest of the switch, and may even be further isolated, 

 beyond the insulation, by a branch of the internal shield. 



