130 THE ROYAL SOCIETY OF CANADA 



No pressure-resistance curves have as yet been obtained for the 

 various substances when in an oil bath, but the absolute pressures 

 have been determined. The table given in this paper shows that in 

 every case the contact pressure was lessened when the contact wires 

 were at first placed in the* bath. But an increase took place very quick- 

 ly in many cases. In others, for instance nickel, the pressure remained 

 constant at 50 mgms. during a period of more than an hour. So that 

 the effect of the oil bath is to produce in some contacts a temporary 

 diminution of the pressure, while a permanent decrease seems to be 

 the effect in other contacts. 



Effects of Cleaning Surface of Conductors. 



All the wires used in the preceding determinations were care- 

 fully cleaned with very fine emery paper and chamois. As indicated 

 by the curve for cleaned copper, 40 to 60 milligrams of pressure will 

 produce a contact of low resistance. To ascertain the comparative 

 pressure for clean and unclean wires, a copper wire, such as may be 

 found in any laboratory, was tested. The consecutive pressure 

 values obtained were approximately 200, 90, 80 and 70 grams, but 

 65 grams would not produce good contact even after a score of 

 observations. 



Conclusions. 



These results as given above for the cleaned wires of metals and 

 alloys seem to substantiate the theory that contact resistance is largely 

 due to a "cushion" effect produced by the formation of a sub-chemical 

 film on the surface of the substance, which film in some cases ultimately 

 becomes the oxide. There are various reasons for drawing such a 

 conclusion. First, the resistance of several substances increased an 

 appreciable amount after an hour or two had elapsed from the time 

 the first observation was made. Secondly, the resistance of the con- 

 tact was greater in changing from oo to than from to oo . The ex- 

 planation is that when the surface film has been broken through or 

 "punctured," the resistance is lessened by a considerable amount. A 

 thorough investigation of this point has not been completed as all 

 the observations were made before the results were examined. Hence 

 in some cases only one curve has been obtained, nearly all the readings 

 having been taken when the resistance was changing from oo to 0. 

 Thirdly, the oil bath prevented the surface film from forming as 

 quickly as in air. Therefore the contact pressure in oil was at first 

 lower than the values obtained when the contact was exposed to the 



