610 White — Switch for Delicate Measurements. 



Art. XXIX. — Note on a Universal Switch for Delicate 

 Potential Measurements; by Walter P. White. 



Two years ago a description was published in this 

 Journal 1 of a universal switch for thermoelectric and 

 other delicate work. The instrument was then a combi- 

 nation of two features which had only been tried sep- 

 arately. One was the contact between thin leaves of 

 copper with sheet celluloid insulation; the other, the 

 mechanical arrangement for making the contacts simply 

 and for changing the combination quickly. The actual 

 instrument has now had over a year of trial ; the cellu- 

 loid insulation has again proved completely satisfactory ; 

 the mechanical arrangement has been as convenient and 

 reliable as was expected; but two modifications have 

 been found advantageous, which deserve publication. 

 To aid in describing them two diagrams are reproduced 

 from the original article, which contains a detailed 

 account of the construction. One modification concerns 

 the contacts between the "auxiliary-connecting frames" 

 and their bus bars. In the earlier apparatus the only 

 contacts were like the contacts between the thermoele- 

 ment leads and their bus bar in the present one, and the 

 making of these contacts evidently involves a slight 

 amount of rubbing. The satisfactory performance of 

 the earlier apparatus must have been promoted by this 

 rubbing. The similar contacts in the present one are 

 equally satisfactory. But the auxiliary-connecting con- 

 tacts, which occurred between two bodies moving in the 

 same straight line, required frequent cleaning. It there- 

 fore seemed best to introduce a little rubbing here. This 

 at once made these contacts as satisfactory as the others, 

 and is by all means to be recommended. It was done by 

 making the farther end of the auxiliary-connecting bus 

 bar travel on an inclined plane (laid under each end of 

 the rod, E), while the sliding frames still move horizon- 

 tally. Friction against the "fixed auxiliary-connecting 

 frame," against which the bus bar returns, could be pro- 

 vided by giving this frame a little horizontal travel, with 

 a spring to press it toward the bus bar. The total travel 

 of the operating rod is increased by the same amount. 



