[reii.i.eyI CONTACT RESISTANCE OF METALS AND ALLOYS 



129 



Results. 



The following table indicates the substances examined, the 

 absolute pressure required to make good contact (a) in air (b) in an 

 oil bath, and the residual resistance:— 



An examination of the preceding table shows that the pressure 

 required to produce good contact in the case of the metals is in general 

 less than that of the alloys. The exceptions so. far as these experi- 

 ments have been carried out are manganin, nickel, gold and aluminum. 

 Manganin and goldplated manganin rank with the metals, requiring 

 a pressure somewhat less than 50 mgs., while a pressure of 180 

 grams was necessary to produce a contact of small resistance for 

 aluminum. Fig 3 gives the curves for gold. Curve No. 1 shows that 

 about 100 milligrams is the good contact pressure. It was observed 

 at the time when the gold was being examined that the atmospheric 

 humidity was above 70 %. This may account for the high pressure 

 value obtained. Curve No. II is the result of observations made 

 about twelve hours later. It shows that the resistance has increased 

 a considerable amount and also that the contact pressure has changed. 



Fig. 4 gives the curves for aluminum, indicating the very high 

 contact pressure of 150 to 180 grams referred to above. These curves 

 also show that the resistance of the aluminum has increased with time, 

 curves Nos. II and III being the results of observations taken one 

 or two hours later than those for curve No. I. Fig. 4 also includes 

 curves for several alloys and nickel. It will be noticed that there 

 are two curves for platinum-silver, one being that obtained as the 

 contact resistance was reduced from oo to 0, and the other in changing 

 the resistance from to oo. As can be seen, these two curves show a 

 difference of resistance for given pressures. Fig. 5 contains two curves 

 for fery wire indicating the same phenomenon of unilateral conduc- 

 tivity. 



