300 
PHYSICS: P. W. BRIDGMAN 
Proc. N. A. S. 
this will be evident from an inspection of figure 1 . It is to be remembered 
that a bridge is an instrument for balancing potentials in different parts 
of a net -work. At D. C. balance the potential drop corresponding to 
tan0 in figure 1 is measured and at A. C. balance the potential drop 
corresponding to tan0'. 
It is evident that the direct effect of the unknown rise of temperature 
is eliminated by this method, because the temperature of the conductor 
is the same to both the direct and alternating current. There is, however, 
an indirect effect which is important, and which must be eliminated. 
Under the joint action of the direct and alternating current the wire 
receives a comparatively large supply of heat steadily, with a small heating 
and cooling effect superposed. Under this superposed heating and cool- 
ing the wire experiences alternations of temperature which give 
rise to fluctuations of resistance. The large direct current flowing through 
the fluctuating resistance gives rise to an alternating difference of poten- 
tial in one of the bridge arms, which affects the A. C. balance. This 
action is like that of a microphone. It becomes vanishingly small at high 
frequencies of the alternating current, because the fluctuations of tem- 
perature become vanishingly small under these conditions. 
The "microphone" effect was eliminated by making readings at a 
number of frequencies, plotting against the reciprocal of frequency and 
extrapolating to zero (that is, infinite frequency). The range of fre- 
quencies employed was from 320 to 3750 cycles per second. The extra- 
polation is therefore over a range only one- tenth of the observed readings. 
As further adding to the certainty of the extrapolation, it may be shown 
by a dimensional argument that the curve extrapolates to zero as a straight 
line. The extrapolated difference between D. C. and A. C. resistance 
gives the sought for departure from Ohm's law. 
Measurements were made on gold and silver. These metals were in 
the form of thin leaf, cemented to a glass backing, cut into the shape of a 
narrow isthmus at the part intended to carry the high current density, 
and cooled by a stream of distilled water flowing over the glass. Two 
thicknesses of gold were used, 8X 10 ~ 6 and 1 . 67 X 10 ~ 5 , and one thickness of 
silver, 2 X 10 ~ 5 cm. Greater thicknesses of gold were tried, but good results 
could not be obtained. It was possible to reach current densities up to 
about 5X10 -6 amp. /cm 2 . 
Further details of the experiment and of the electrical arrangements, 
which were sufficiently simple and obvious, will be described elsewhere, 
probably in the Proc. Airier. Acad. Arts, and Sci. 
All of the experimental results obtained on eleven different samples 
of 8X10" 6 gold (three of these were films formed by cathode deposit) are 
collected in figure 2. The ordinates are the extrapolated difference 
between D. C. and A C. resistance in terms of the initial resistance, and 
