W. P. White — Switch for Thermoelement Work. 309 



The electrical arrangements figured here are those appro- 

 priate to a four-dial combination potentiometer, with eight 

 connections for various " unknowns," but can easily be modi- 

 fied for other methods of potential measurement. The type 

 of contact used is not suitable for resistance measurements of 

 the highest precision, unless by methods which eliminate the 

 resistance of the contacts. Several features of the arrange- 

 ment could well be used for resistance measurements, but we 

 have had no occasion to develop any apparatus for such. 



The leading features of the switch are the following ; 



1. The contacts are made between strips of thin sheet cop- 

 per. This arrangement probably gives as small parasitic elec- 

 tromotive forces as any known, and is evidently among the 

 first also in ease of construction. 



2. Sheet celluloid* is used exclusively for insulation, laid 

 over wood where solidity is necessary. This also makes for 

 very low cost of construction, by saving the time needed to 

 work hard rubber. The copper strips are held in place by 

 tacking celluloid strips over them (keeping the tacks away 

 from the copper) or bending the copper over the edges of the 

 celluloid, using shallow notches in the edges of the celluloid, or 

 slits or holes through it, to keep the copper located. Further 

 details on this point seem unnecessary. 



3. The contacts are made by pressure alone, f Hence when 

 the pressure is released the moving parts will return to their 

 resting positions if light springs are provided for the purpose. 

 It is sufficient, therefore, in devising any scheme of connec- 

 tions, to arrange for exerting pressure at several points, and 

 this enables an exceedingly convenient system of connections 

 to be very easily obtained. 



4. The convenient system of connections in the present case 

 is obtained as follows : A straight wooden rod, when pressed 

 in, presses against the " unknown " (say the thermoelement) 

 contact, closing that. At the same time, a pin, projecting 

 sidewise from the rod, pushes a frame which carries the aux- 

 iliary (usually the potentiometer) contacts. A turn of the rod 

 makes the pin strike a different frame, giving a different 



* Celluloid does not enjoy the best reputation as an insulator, but I found 

 in two samples of sheet celluloid 0'3 mm thick, one over 5 years old, in damp 

 weather, a resistance certainly exceeding 600 megohms between tightly 

 pressed metal conductors of 7 8 i cm area. Only in air of extreme saturation 

 have I observed detectible surface leakage with it. Of course thin hard rub- 

 ber can be used like the celluloid, and even with hard rubber bars the 

 switch can not be considered elaborate in view of what it can do. 



f The question has often been raised whether pressure, with practically no 

 friction, would give a reliable contact. In five years' experience I have uni- 

 formly found that it does, with the surfaces cleaned certainly not oftener 

 than once in three months. The pressure was several hundred grams on each 

 contact surface of about 6 x 20 mm , but less would probably have answered. 



