The technique adopted in some experiments employing Geiger-Mueller counters is described. The behaviour of a counter in which the extinction of the discharge is external is contrasted with behaviour of a counter in which extinction is internal and dependent on the addition to the gas of some organic vapour such as alcohol. It is shown that the internally extinguished type of counter may be used with a resistance of the order 50,000 ohms in series with it while still retaining all its useful features.

Methods of construction of internally extinguished counters are described, particular attention being paid to the question of developing a thin-walled type suitable for experiments employing the method of coincidences. The effect of using different wall materials and gases is investigated with a view to finding those most suitable.

The measurement of the quantum energy of hard γ-radiation by the method of coincidences is discussed and details of a reliable two-stage amplifying circuit with a resolving time T of 10−6 sec. are given. Various factors which may influence the value of T are noted.

In the preceding paper, Blackett and Williams described an automatic curve follower for use with the differential analyser. The device would only operate if the x-coordinate was either continuously increasing or continuously decreasing; it could not handle input functions where the x value pulsated, as may often happen when the x-coordinate is not the independent variable.

The absorption of the harder γ-rays from ionium has been studied, the first, of energy 68 ± 1 k.e.V., by the method of bracketing, making use of the K absorption discontinuities of tantalum and tungsten; the second, of energy 190 ± 20k.e.V., by analysis of the absorption curves. The intensity of these γ-rays is estimated to be 1 quantum in 10−3 disintegration for each γ-ray.

By “sensitive” time of a cloud chamber is meant the length of time for which the supersaturation in the chamber remains sufficient to cause condensation along the tracks of ionizing particles. If the cooling expansion is made rapidly this sensitive time depends on the rate at which the temperature of the gas in the chamber rises after the expansion. This problem of the warming of a gas in a vessel, after an initial cooling, has been previously considered by Rayleigh (1). To follow the whole history of the warming until the gas reaches the temperature of the vessel requires an elaborate mathematical treatment which can only be carried out for specially shaped vessels. Rayleigh considers two special cases, viz. a gas contained between two parallel infinite planes, and a gas contained in a spherical vessel. The results obtained by Rayleigh for these cases are, however, not very helpful in dealing with the cloud-chamber problem under consideration. This is so, not so much because specially shaped vessels are assumed, but because the treatment does not lend itself to an evaluation of the rate of warming in the early stages of the process. It is this initial warming that we are concerned with here, since a cloud chamber ceases to be sensitive when the average temperature of the gas has risen by only 5% or less of the cooling produced by the expansion. Rayleigh's result for the average rise in temperature of the gas at any instant is represented by an infinite series which converges very slowly when the rise in temperature is small.

The irreversible heating of iron ammonium alum by an alternating field is shown to be proportional to the frequency of the field. The heating is therefore similar to that arising from hysteresis in ferromagnetics, but no remanence could be found by a ballistic method at the lowest temperature obtained, about 0·075° K. The rate of heating was much increased by the superposition of a steady magnetic field of a few hundred gauss.