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



20 mm above the center line, in order to clear the locking bar; 

 hence the iron rod is 3 mm nearer the bottom in this strip than in 

 the 20 mm strip. 



Between the back ends of the upper and lower auxiliary con- 

 necting frames is the fixed auxiliary -connecting frame FX 

 (fig. 2) which is merely a single square wooden strip provided 

 with 4 insulated copper strips like the others. Like the rod 

 frame it is held in place by two steel pins in the base, and can be 

 lifted off at any time. 



Contact with the auxiliary-connecting frames is made by the 

 auxiliary bus bars (fig. 2). This arrangement consists of two 

 broad wedges, 8 c,n long, borne on a wooden cross-piece, which 

 rocks on the wedge-shaped front of another flat piece of wood. 

 The two wedges carry (with suitable insulation) 4 strips of 

 copper, opposite to those on the frames, and running to the 

 battery and galvanometer terminals to which the potentiometer 

 switches are to be connected. The whole is pressed forward by 

 the rod, R, which is pulled by two long springs. The object of 

 the two wedges and rocking cross-piece is, of course, to divide 

 the pressure equally among the 4 contacts. The combination is 

 fastened together as follows : Through the cross-piece, opposite 

 the apex of each of the 3 wedges, are bored 2 holes, large enough 

 to be an easy fit for a small nail. Each hole is then counterbored 

 nearly through so as to take the head of the nail easily. The 

 nails are then driven through into the wedges, far enough to hold 

 these in place but still permit the slight rocking motion required. 

 Pressure against the wedges of course causes no wear or other 

 injury to the holes. 



When the movable auxiliary-connecting frames are out of 

 action, the contact strips on them are about 4 mm to the front (left, 

 in the figure) from those on the fixed auxiliary-connecting frame, 

 so that this frame receives the pressure of the bus bars, and is 

 thereby connected in. When either of the movable frames is 

 pushed in. it pushes the bus bar away from the stationary frame, 

 breaking that connection at the same time as it establishes its own. 

 Thus with 2 movable frames three sets of connections are possible. 

 When the moving frame reaches the bus bars two sets of connec- 

 tions are joined parallel for an instant, but this does no harm in 

 the case of a potentiometer. If in any case this momentary paral- 

 lelling of connections should be deemed undesirable, it could be 

 avoided by a different, probably slightly less convenient, arrange- 

 ment of the contacts. The bus bars must be moved about 3 mm to 

 clear the stationary frame freely, and after that 8 mm further while 

 the thermoelement contact is being established. The springs 

 must be long enough so that this motion of n mm will not greatly 

 increase the pressure exerted. 



The operating rods move 10 mm before engaging the movable 

 auxiliary-connecting frames. Hence these frames, at rest, are 

 10 mm from the resting position of the impelling pins, P 2 , of the 

 operating rods. This 10 mm of motion is used to raise the locking 



