46 
PHYSICS: DUANE AND MAZUMDER 
Proc. N. a. S. 
efficient of absorption and the wave-length may be expressed approxi- 
mately by an equation of the form 
!^ = k\' + s. (2) 
P 
Hull and Rice give for the constants k and s: k = 14.9 and s = .12 for alu- 
minium; and ^ = 150 and 5 = .12 for copper, the range of wave-lengths 
lying between .392 and .147. 
Richtmyer gives the following values for the constants k and 5 ; for alu- 
minium fe= 14.5 or 14.3 ands = .15 or .16; and f or copper fe = 147 and 5 = .5. 
2. As a great deal of work is now being done on X-rays of wave-lengths 
shorter than .135, we have undertaken to extend the measurements of the 
absorption in aluminium and copper to cover the range of these shorter 
waves. We have measured the mass coefficients of absorption for alumin- 
ium and copper for wave-lengths from .165 to .095. 
In making these measurements we have used a Coolidge X-ray tube 
with a Tungsten target, excited by the high tension, constant voltage 
generating plant that has been described elsewhere. The constant 
voltage employed for these short waves amounted to about 160,000 volts. 
The X-ray spectrometer employed has been described also, together 
with the methods of protecting it against stray radiation, etc. This 
protection against stray radiation becomes of very great importance 
for the shorter, penetrating rays. It is sufficient to call attention to the 
fact that the X-ray tube with its generating plant was in one room and 
the spectrometer in an adjoining room, and that thick lead plates were 
fastened to the wall between the two rooms for additional protection. 
Further, the entire spectrometer was in a zinc box that completely sur- 
rounded it. 
3. In making measurements of the coefficients of absorption we set the 
spectrometer to reflect a given wave-length into its ionization chamber. 
We then measure the ionization current with a plate of the absorbing 
substance placed between the X-ray tube and the hole in the wall through 
w^hich the X-rays pass to the spectrometer. We next measure the ioni- 
zation current with this plate removed. From each of these currents 
must be subtracted the current due to scattered radiation and to the 
natural leak in the instrument. This correction we estimate by turning 
the crystal of the spectrometer into such a position that it does not reflect 
rays into the ionization chamber, and measuring the ionization currents 
both with and without the absorbing plate in place. Subtracting the cor- 
rection currents from the currents obtained with the crystal in its proper 
position, we get the fraction of radiation passing through the absorbing 
substance, and from this we calculate the mass coefficient of absorption 
in the usual way. 
