238 
PHYSICS: DUANE, PALMER AND YEH 
Proc. N. A. S 
made the most accurate measurement of h in our laboratory. Using 
Millikan's value of e = 4.774X 10 " 10 they obtained the value h = 6.555 
xio- 27 . 
The object of the research reported in this note is to increase the accuracy 
of the measurement of h. We use a new and somewhat improved spectrom- 
eter and a new calcite crystal. The X-ray tube contains a tungsten 
target and a Coolidge cathode. A side arm attached to the tube extends 
out toward the spectrometer and carries at its outer end a thin mica 
window. The increased intensity of the X-rays coming through this 
window enables us to use a narrower spectrometer slit, which red uces the 
correction that must be made for the slit's width. 
As in the previous researches the high tension storage battery supplies 
the current through the X-ray tube. In the present research we have 
greatly increased the accuracy of the measurement of the difference of 
potential applied to the tube. Whereas in the previous researches this 
difference of potential was compared with the electromotive force of a 
standard cell through the calibration of several intermediate instruments 
(an electrostatic voltmeter, an ammeter, and a potentiometer), we now 
compare the difference of potential directly with the electromotive force 
of a standard cell by means of the simple potentiometer method. In this 
way we eliminate the errors in the calibrations of the various instruments. 
The main circuit in the simple potentiometer consists of a large number of 
coils of manganin wire having a total resistance of over six million ohms. 
We use the same precautions in insulating the various circuits as one 
employs in making measurements of ionization currents. The ratio of 
the resistances of two sections of the main circuit gives us directly the 
ratio of the difference of potential to the electromotive force of the standard 
cell. As the ratio of two resistances can be measured with extreme ac- 
curacy, we think that we know the difference of potential applied to our 
X-ray tube with about the accuracy with which the electromotive force 
of the standard cell has been determined. We use two unsaturated 
Weston standard cells, each of which has been tested at the Bureau of 
Standards, and we compare them with each other from time to time. 
The certificates from the Bureau give the electromotive forces of the cells 
to within one part in ten thousand. 
The actual drop in potential in an X-ray tube through which the electrons 
fall differs from the drop in potential measured by the potentiometer by a 
small amount due to the Volta effect, the current that heats the coil of 
wire in the Coolidge cathode and the high temperature of the wire. In 
our experiments the circuits are so connected and the voltage applied 
to the tube is so high that we may neglect the corrections due to these 
effects. 
