McLeod Gauge Research Articles

Studies with the Ionization Gauge. I. Construction and Method of Calibration

An Ionization Gauge for Measuring Very Low Pressures.---This gauge is the result of an attempt to develop a more convenient instrument than those previously available. It is based upon the fact that the positive ionization produced by a given stream of electrons varies with the gas pressure. By adopting a construction similar to that used for low power pliotrons, erratic effects due to the charging up of the glass walls were avoided and, with a constant electron emission, a linear relation was obtained between ionization current and pressure down to the lowest pressure actually obtaiued, about ${10}^{\ensuremath{-}3}$ bar. For pressures above ${10}^{\ensuremath{-}1}$ bar the calibration was carried out by comparison with a McLeod gauge; but for lower pressures a method was used which involves an application of Knudsen's laws of flow. Characteristic curves showing the effect of varying the positive voltage and the electron emission on the ionization current are given. The low limit of pressure measurable with such a gauge is fixed only by the sensitivity of the galvanometer used to measure the ionization current. With an ordinary sensitive galvanometer, pressures as low as ${10}^{\ensuremath{-}4}$ bar can readily be determined.

To Measure the Pressure in a High Vacuum by Observation of Logarithmic Decrement

In experiments on the Newtonian Constant (Phil. Trans., May, 1916) the author used a torsion balance in a vacuum which varied in different cases from 15 mm. to 0.00001 mm. pressure. Before sealing the vessel the pressure was determined by a McLeod gauge. Values of the pressure after sealing off were deduced, in the case of the higher vacua, from observations of the damping of the torsion system. The formula employed is due to the late Prof. Poynting, and can be expressed in the form P = 35.6(I/saT)λ where I=moment of inertia of suspended system, s=area of surface (supposed plane) which is experiencing the resistance, a=mean distance of plane from centre of rotation, T=period of oscillation, and λ=the observed logarithmic decrement. A table and curve are given showing the relation between P and λ.

Experimental Evidence for the Essential Identity of the Selective and Normal Photo-Electric Effects

In the fall of 1913, while studying the photo-electric properties of freshly cut surfaces of the alkali metals in extreme vacua, we observed that immediately after first cutting, the fresh surface of sodium showed very large photo-sensitiveness when tested with monochromatic light of wave length 5461 A., even when the vacuum was of the order 10(-6) mm. as measured by a McLeod gauge. But after several weeks of experimenting and many cuttings a condition was reached in which a freshly cut surface was completely insensitive when illuminated with this wave length. The lost sensitiveness reappeared, however, in the course of not more than two minutes after cutting, and grew rapidly to a very large value in fifteen or twenty minutes. When the gas pressure was of the order of 0.01 mm. the same phenomenon occurred but the rise to a maximum value was less rapid. From these results we began to surmise that photo-electric currents must be due to the influence of some active gas, which diffused from the walls to the metal and whose action upon the surface was retarded by the presence of an inert gas.

Production of solid oxygen by the evaporation of the liquid

It seems strange that while liquid hydrogen or nitrogen can easily be changed into the solid condition by evaporating the respective liquids under exhaustion, yet the ordinary air pumps fail to effect the transition of state in the case of liquid oxygen. This is due to the small vapour pressure of solid oxygen at its melting point, as compared with hydrogen or nitrogen under similar conditions; together with the greater need of very perfect heat isolation in the arrangement of the experiment. By means of the use of charcoal as a gaseous condensing agent at low temperatures, combined with the employment of proper vacuum vessels, the change from the liquid into the solid can be easily effected. Pure liquid oxygen, contained in a properly isolated vessel, subjected to the exhaust produced by a quantity of charcoal kept at about the temperature of boiling oxygen, has its pressure lowered sufficiently to produce solidification to a transparent jelly. The pressure at which solidification takes place was determined by connecting a McLeod gauge to the vessel containing the solid oxygen. About 75 grm. of good coconut charcoal is necessary to produce and maintain the necessary conditions of exhaust. From 5 to 10 c. c. of liquid oxygen were employed, previously exhausted by an air pump. The oxygen exhibits considerable supercooling, and a pressure of less than half the melting pressure can usually be maintained on the liquid without producing solidification. These experiments indicate that the melting point pressure is between 1·115 and 1·125 mm.

Societies and Academies

LONDON. Royal Society, Apiil 28.—Sir Archibald Geikie, K.C.B., president, in the chair.—R. B. Sangster: The rotatory character of some terrestrial magnetic disturbances at Greenwich and on their diurnal distribution. The paper commences with an investigation of the changes in direction of the line of total magnetic force at Greenwich on 1903 October 12d. 18h. to 23h., when a considerable magnetic disturbance was in evidence. Measurements of the published registers of all three force components were made at equivalent time intervals of about five minutes, whence is obtained a diagram showing the variation of the force component perpendicular to the line of total force. The diagram shows there was an almost wholly rotatory motion of the transverse disturbance vector, the trace consisting of six distinct convolutions varying greatly in size, but consistent in anticlockwise progression. Several other disturbances during epoch 1900–7 are examined in detail, and it is shown that a right- or left-handed rotatory character in the motion of the disturbance vector was of fairly frequent occurrence, while change from left to right not uncommonly occurred about midnight. It was also found that the same direction of rotation often persisted for several hours, and tables of the diurnal distribution of right- and left-hand rotatory disturbance are furnished to show that those of right-hand character were entirely absent during the hours 4 p.m. to 9 p.m., while, meantime, the left-handed rotations were very prevalent, and reached a notable maximum at 8 p.m. Other points in the diurnal distribution are noted, including the more decided effect resulting from a seasonal grouping of the seventy disturbed days dealt with.—D. Orson Wood: The liberation of helium from minerals by the action of heat. Experiments were made to determine how the volume of helium liberated from radio-active minerals by the action of heat depends on the temperature, and on the time for which that temperature is maintained, in particular with the view of the future use of heat to release all the helium contained in minerals not easily treated by chemical methods. The minerals experimented on were monazite and thorianite, the one comparatively poor and the other very rich in helium. The ground minerals were heated, in vacua, in tubes of Jena glass or quartz, by an electric heater consisting of a single coil of nickel wire, to temperatures up to 1200° C., which were measured by a Pt resistance thermometer or a Pt Pt-Rh thermocouple. The gas released was purified by drawing it through KOH and P2O5 tubes, and finally by Na —K electrodes. The volume was measured in a modified McLeod gauge (described by Prof. Strutt, Proceedings, vol. Ixxx.) specially constructed for the measurement of volumes over a large range—i c.c. to i c.mm. Curves are given to show the volume of helium liberated with time at constant temperatures (250°–1000° C.), and also the percentage of the total content obtainable after prolonged heating at the different temperatures. The way in which the gas must be supposed to be retained within the mineral to accord with the results obtained is discussed, and it is concluded (1) that heat may be used for the complete liberation of the gas if a sufficiently high temperature (about 900° C.) is reached, and (2) that the results are in agreement with the supposition that a small proportion of the gas is diffused through the mineral and that the remainder is concentrated in very minute cavities within it.—Prof. Swale Vincent: The chromophil tissues and the adrenal medulla. The author gives an account of the gross anatomy and histology of the chromophil tissues in mammals, and especially in the dog. Descriptions and drawings of the groups of cells in the sympathetic ganglia and of the chromophil bodies in other regions are furnished, and comparisons are made between their structure and that of the adrenal medulla. An extract of the abdominal chromophil body of the dog has precisely the same powerful effect upon the blood pressure as an extract made from the medulla of the adrenal. There seems no reason why one cannot admit the hypothesis that all the chromophil cells have an internal secretion, though this process is more completely elaborated in the larger chromophil bodies and in the adrenal medulla.

Long-period determination of the rate of production of helium from radium

In a previous communication the rate of the production of helium from 70 milligrammes of radium chloride was determined by a succession of observations on the growth of pressure measured by a McLeod gauge. These observations extended over a period of about six weeks. It was thought desirable to make an experiment to determine the amount of helium resulting from this same sample of radium, after standing in a sealed bulb for an extended period. For this purpose the bulb containing the radium chloride was sealed off at the conclusion of the above-mentioned experiment of 1908 and kept for nine months. In order to measure the helium thus produced it was necessary to devise a vacuum-tight joint between the sealed radium bulb and a McLeod gauge so constructed that, after thoroughly exhausting the gauge, the drawn-out end of the radium bulb could be broken off, thus allowing the pressure of the accumulated helium in the radium bulb to be rapidly determined. Such a joint is shown in fig. 1. A is the small bottle containing the radium chloride resting in the cylindrical bulb B with long tubular neck drawn out to a capillary, and sealed off at C at the conclusion of the first experiment. This tubular neck slides with slight clearance into the drawn-out neck of another cylindrical bulb D of stout glass, and is attached to it by a short, stout, indiarubber connection protected by a glass mercury cup M, previously fixed as shown on the neck of the radium bulb. This drawn-out neck is so shaped that the radium bulb can be readily rocked in a vertical plane about the neck as pivot within the limits of the diameter of D when so connected.

Societies and Academies

LONDON. Roval Society, January 27.— Sir James Dewar: Long-period determination of the rate of production of helium from radium. In a previous communication the rate of the production of helium from 70 milligrams of radium chloride was determined by a succession of observations on the growth of pressure measured by a McLeod gauge. These observations extended over a period of about six weeks. It was thought desirable to make an experiment to determine the amount of helium resulting from this same sample of radium, after standing in a sealed bulb for an extended period. For this purpose the bulb containing the radium chloride was sealed off at the conclusion of the above-mentioned experiment of 1908 and kept for nine months. In order to measure the helium thus produced it was necessary to devise a vacuum-tight joint between the sealed radium bulb and a McLeod gauge so constructed that, after thoroughly exhausting the gauge, the drawn-out end of the radium bulb could be broken off, thus allowing the pressure of the accumulated helium in the radium bulb to be rapidly determined. The total volume of the apparatus was 320 c.c. The pressure in the radium bulb when sealed off at the conclusion of first experiment was 0.00406 mm., the partial pressure due to this amount of helium would be 0.00008 mm., which must be deducted from the observed pressure to get the true pressure due to the helium produced in the radium bulb during the period in which it remained sealed up; also the pressure in the gauge, before breaking (0.00005 mm.), must also be deducted. This gives a corrected pressure of 0.01613 mm., obtained after heating the salt, due to the helium produced from 70 milligrams of pure radium chloride during a period of 275 days, in a space the total volume of which was 320 c.c. The value of the rate in terms of cubic millimetres of helium per gram of radium per day is thus deduced as 0.463.

Note on the use of the radiometer in observing small gas presures; application to the detection of the gaseous products produced by radio-active bodies

Note on the use of the radiometer in observing small gas presures; application to the detection of the gaseous products produced by radio-active bodies

On the velocity of rotation of the electric discharge in gases at low pressures in a radial magnetic field

The following paper contains an account of a series of experiments on the motion of an electric discharge in a magnetic field perpendicular to the direction of the discharge current. The fact that the discharge moves in a magnetic field like a flexible conductor carrying a current was discovered long ago, and De La Rive showed that it could be made to rotate continuously round one pole of a magnet placed inside the vacuum tube. The apparatus used in the present experiments was similar in principle to De La Rive’s, but was arranged so that fairly exact measurements of the various quantities concerned could be obtained. Fig. 1 shows a vertical section of the vacuum tube and magnet used. The tube consisted of two concentric glass tubes cemented with sealing wax into aluminium discs. The discs had grooves turned in them to fit the glass tubes, and the part of the discs between the tubes projected a few millimetres, so that there was no danger of the discharge passing through the sealing wax. Polished platinum rings were fixed on to the aluminium discs between the glass tubes, and these formed the electrodes between which the discharge was passed. The ends of the tubes were carefully ground truly perpendicular to their axes, and the two platinum rings were accurately parallel. To keep the electrodes cool, a ring of narrow brass tubing was soldered on to the back of each disc and a stream of water was kept flowing through these whenever a discharge was passed. This arrangement enabled comparatively large currents to be used without softening the sealing wax. A narrow copper tube was soldered into one of the discs and communicated with the interior of the vacuum tube through a fine hole. The tube was connected by a mercury sealed joint with a glass tube leading to a bulb containing pure phosphorus pentoxide, a Töpler pump, and a McLeod gauge.