Studies in Superconductivity II. Evaporated Lead Films

The electrical and magnetic properties of evaporated lead films of several thicknesses (1000-3000A) have been studied in the superconducting state. The films exhibited transition temperatures (in zero magnetic field) of 7.23±0.03°K irrespective of thickness. The disappearance of resistance (99 to 1 percent) usually occurred within a range of 0.10°. The depression of the transition temperature with increase in measuring current $I_c$ was found to be greater the thinner the film, all films showing larger depressions than a 254-micron lead wire ($T_c=7.20\mathrm{}$°K). The depression of the transition temperature in several magnetic fields (0-80 oersteds) perpendicular to the plane of the film was found to depend upon thickness, the thinner films requiring larger external fields to produce a unit depression. The $I_c \mathrm{versus} T_c$ curves obtained exhibit inflection points not heretofore observed, while the $H_c \mathrm{versus} T_c$ curves obtained suggest the existence of a similar phenomenon. The effect of measuring current on the depression of the transition temperature in constant magnetic fields also has been studied.