6o8 
PHYSICS: DUANE, FRICKE AND STEN STROM Proc. N. A. S. 
ganese (atomic number 25).^ De Broglie/ and Siegbahn and Jonsson^ 
have published measurements of the K critical absorption by means of 
photographic spectrometers for the chemical elements of high atomic 
numbers included in this research. They examined spectra of the first 
order. Columns 3 and 4 in the table contain the values of the wave- 
lengths they give. 
Apparatus and Method. — The general method of making the measure- 
ments does not differ essentially from that employed by Duane and 
Shimizu.^ We have been able, however, to apply a somewhat higher 
voltage to the X-ray tube than in the earlier researches. The X-ray tubes 
we have used do not withstand a constant difference of potential greater 
than about 115,000 volts. To produce X-rays shorter than the K series of 
uranium, however, requires a voltage considerably in excess of this figure. 
By encasing the arms of the X-ray tube and also those of the kenotrons 
belonging to the high tension generating plant in oil baths we have been 
able to excite the tube with approximately constant voltages up to about 
140,000 volts. We estimated these voltages with an electrostatic volt- 
meter, which we calibrated by measuring the current from the generating 
plant through a series of coils of manganin wire having a total resistance 
of 6.043,000 ohms. As a check on the voltage measurements and on the 
constancy of the difference of potential we determined the short wave- 
length limit of the general X-ray spectrum and calculated the voltage 
by means of the quantum equation Ve = kv. Duane and Hunt^ showed 
by experiments that this law holds for the limit of the spectrum, and 
their results have been verified by more recent researches. 
Results of the Measurements. — On platting the currents in the ioniza- 
tion chamber against the angular positions of the table on the spectrom- 
eter that supports the crystal we obtain curves, examples of which 
appear in figures 1 and 2. The sharp drops, a, in these curves, represent 
the critical absorption due to the chemical element under investigation. 
A layer of matter containing this element is placed in the path of the 
X-rays between the X-ray tube and the spectrometer. The angular dis- 
tance between the mid points in corresponding drops on the two sides 
of the zero gives us twice the glancing angle, B, which must be substituted 
in the usual formula, 
\ = 2d sin 6 
to calculate the critical absorption wave-length. A correction for excen- 
tricity, amounting to about 27" of arc, has to be subtracted from the val- 
ues of 6 measured on the graphs. 
One pair of curves in figure 1 represents the critical absorption of 
uranium. These curves are of special interest, for they correspond to 
the shortest characteristic X-ray (of any chemical element) that has been 
discovered up to the present time. The centres of the drops can be 
