[REILLEY-HENRY] CONTACT RESISTANCE IN OIL 149 
residual pressure necessary to reduce the contact resistance to zero. 
In the table of pressures already given is included a column under 
the heading ‘“‘residual resistance.” This residual resistance is the 
contact resistance, which could be lessened by the application of very 
large pressures as compared with the contact pressure necessary to 
reduce the resistance to what is called ‘‘residual resistance.” 
The first part of this investigation consisted in the determination 
of contact resistances for platinum, silver, copper, nickel, manganin, 
constantan, and fery-wire in baths of gasoline, using the wires in the 
order in which they are mentioned. In the case of platinum if the 
residual resistance was not included, then the pressure was 45 mgs.; 
if the residual resistance was included, the pressure was 82 mgs. 
This result is consistent with the value obtained for platinum in 
kerosene. The contact pressure for silver shows similar values for 
kerosene and gasoline. In the case of copper, as indicated in the table 
already referred to, there was no residual resistance. With the con- 
tacts in air the required pressure was 60 mgs. in air, 50 in kerosene 
and in gasoline 65. The values of the contact pressures for nickel in 
air, kerosene and gasoline show the resistance effects of oil baths, in 
air the pressure being 140 mgs., in kerosene 50, and in gasoline 43-8. 
For manganin the values of contact pressure in kerosene and gasoline 
(if the necessary residual resistance correction be made) are less than 
in air; the same applies for constantan and fery-wires, the values for 
the former in air, kerosene and gasoline being, 160, 100, 145 mgs. 
respectively; for the latter 200, 165, 137 mgs. respectively. 
The contact pressures necessary to produce zero resistance for 
these various metals and alloys in machine oil and dried machine oil 
were next determined. The purpose of drying the oil was to observe 
the effect of moisture on the resistance of the contacts. The oil was 
dried by filtering several times through large quantities of sodium. 
This drying process was done frequently to make sure that no moisture 
remained in the oil bath. In the bottom of the oil bath were placed 
large pieces of sodium which were allowed to remain there, even while 
the observations were being made. The results obtained show 
without a doubt that the careful drying of the oil lessened the contact 
pressure. In every case for both the metals and alloys the contact 
pressure in the dried machine oil is considerably less than in the un- 
dried oil. 
For platinum in undried machine oil a pressure of 63 mgs. 
produced good contact. This was lessened by 20% in dried machine 
oil; for silver 56 mg. with a decrease of 10% in the dried oil; for 
nickel 84-5 mgs. with a falling off of 25%; for manganin 64 mgs. 
constantan 62, fery-wire 128 with somewhat smaller diminutions of 
