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The equations for low amplitude space‐charge waves in an electron stream containing heavy positive ions are considered. At frequencies a little below the ion plasma frequency there should be a wave which increases in amplitude as it travels (negative attenuation). Ion motion tends to limit the rate of increase. In a beam filling a conducting cylinder and confined by a uniform magnetic field there should be an increasing wave at frequencies near the ion cyclotron frequency. Such waves might explain observed fluctuations in long electron beams.

An earlier paper presented equations for increasing waves in the presence of two streams of charged particles having different velocities, and solved the equations assuming the velocity of one group of particles to be zero or small. Numerical solutions giving the rate of increase and the phase velocity of the increasing wave for a wide range of parameters, covering cases of ion oscillation and double‐stream amplification, are presented here.

In electron beam devices the thermal velocities of the electrons leaving the cathode limit the current density which can be attained with given cathode current density, cathode temperature, potential, and angular spread of arriving electrons. By using Liouville's theorem, limiting expressions are obtained which are valid for any point focus or line focus concentrating system using steady electric and magnetic fields. Current densities in perfectly focused beams are also evaluated. Curves are presented which relate the nearness of approach to the maximum attainable current density with the fraction of cathode current necessarily lost in the concentrating system. Use of the expressions obtained is illustrated by applying them to cathode ray tubes and electron multipliers.

This paper discusses the operation and characteristics of electrostatic photo‐multipliers, using the Western Electric D‐159076 photo‐multiplier as an example to illustrate specific points. The following subjects are discussed: description of operation, advantages and applications, typical operating conditions, variation of sensitivity with voltage, hum due to ripple in power supply, effect of unequal voltage division, output impedance, linearity of response and fluctuation noises.

Elementary arguments of conservation of energy and momentum are used to derive expressions for what Sturrock terms the ``formal'' momentum, angular momentum, and energy of a plane wave in a moving medium. If E is the energy per unit length, p the momentum per unit length, q the angular momentum per unit length, vf the phase velocity, and if the angular variation of a circulatory polarized wave is as exp(—jnθ), p=E/vf,q=nE/ω. If E0 is the energy per unit length observed moving with the medium, and then if the medium moves past the observer with a velocity u and the observer measures a phase velocity vf, he observes an energy per unit length E, E=E0/(1−u/vf). When p, q, and E are not the physical momenta and energy, they lead to the correct power and force for sinusoidal sources moving with respect to the medium.

The operation of a wave‐type parametric amplifier may be thought of in terms of the force exerted by traveling discontinuities or reflecting elements on the waves present in the device. By invoking the principles of conservation of energy and momentum it is possible to understand in very simple terms certain limitations on the operation of such devices and to arrive very easily at certain general relations governing their behavior, including the Manley‐Rowe relation. It is proposed that for a guided wave the momentum per unit distance is the power divided by the product of the group and phase velocities.

Electrodes are devised by means of which rectilinear electron flow according to well‐known space charge equations can be realized in beams surrounded by charge‐free space. It is shown how these electrodes can be used in the design of electron guns having desirable characteristics.

When two lossless modes of propagation are coupled, waves which increase or decrease with distance may arise when the power flow of the two modes is in opposite directions or when power is generated in the coupling device. This behavior is characteristic of wave filters, traveling‐wave tubes, double‐stream amplifiers, and space‐charge‐wave amplifiers. Such behavior is analyzed assuming linearity and conservation of energy only.

It is proposed that if Imax and Imin are the maximum and minimum noise currents in a noise standing wave on an electron stream, then the product of their magnitudes cannot be smaller than |Imax||Imin|=αeI0B(ωωq)(kTceV0). Various assumptions lead to a factor α ranging between 0.8 and 1. I0 is beam current, B is band width, ω is frequency, ωq is effective plasma frequency, Tc is cathode temperature, and V0 is the voltage specifying the velocity of the electrons.

By using a new theorem concerning noise in electron streams together with work by D. A. Watkins, an expression for the minimum noise figure of a traveling‐wave tube with a uniform helix is deduced. For reasonable values of tube parameters this minimum lies around 6 db.

A type of electron flow in a constant magnetic field is described. The beam of electrons is supposed to be everywhere in thermal equilibrium and the usual Brillouin flow is found when the equilibrium temperature tends to zero. Some considerations are put forward bearing on the choice of a suitable beam temperature in specific problems.

Most physicists probably spend most of their professional effort working at physics in a rather narrow, technical sense. For this alone there are scarcely enough hours in the day. Should a pause occur, there are always papers to be read, those which one really should have got around to a long time ago. Indeed, keeping up with the literature could be a fulltime job in itself.