Volume 12, Issue 10, October 1941
Index of content:
12(1941); http://dx.doi.org/10.1063/1.1769782View Description Hide Description
An apparatus is described for accelerating protons up to 400 kv with which to irradiate biological specimens in vacuum. The voltage is supplied by an air‐insulated electrostatic generator used with a multiple section tube. The protons are produced by a convenient and reliable source, which is described. Details of the irradiation of spores of the fungus Aspergillus niger with protons are mentioned.
12(1941); http://dx.doi.org/10.1063/1.1769783View Description Hide Description
The purpose of the present paper is to describe an instrument which is capable of measuring small d.c. voltages (of the order of 10−3 volt full scale) without drawing any but an infinitesimal current, and which at the same time is as easy to use and as reliable as an ordinary d.c. instrument. When properly shunted, it can be used as an accurate, direct indicating ammeter for measuring currents of the order of 10−12 ampere full scale.
12(1941); http://dx.doi.org/10.1063/1.1769784View Description Hide Description
A vacuum tube sweep circuit is described which permits the transit of the oscilloscope beam to cover any desired interval of time during the cycle of events under observation, controllable delays being possible both before and after the transit. The circuit may also be used for single sweep work. A simple method for impressing time base markings is also described. Applications of the circuit are discussed and adaptations to multiple sweep photography are suggested.
12(1941); http://dx.doi.org/10.1063/1.1769785View Description Hide Description
A method is described for measuring the thickness of thin metallic films, using λ=3654A filtered from the mercury arc spectrum as the source of light for the Michelson interferometer. The shorter wave‐length light source produces a proportionally greater fringe shift in the interference pattern and thus thinner films can be measured with a greater accuracy. This method is used satisfactorily for films of the range 150–600A.
12(1941); http://dx.doi.org/10.1063/1.1769786View Description Hide Description
The design and operation of a 12‐stage electrostatic multiplier tube as a photon counter are discussed. The tube has a pure tantalum photoelectric surface, the multiplying electrodes being covered with oxidizedberyllium. With 300 volts between successive electrodes the tube has a background counting rate of about 3 to 5 pulses per minute and records about one photon in 500 at a wave‐length of 10−8 cm. The multiplication is about 18,000.