Again, the radiation from the emanation admitted into D in the above experiment produced such an ionization that, when T was kept at a high potential, D got a charge corresponding to 100 scale-divisions in 47 seconds, with a capacity of 1 microfarad joined to the electrometer.

This charge is produced by ions giving up their charge to D, the charge of each ion being e.

The charge given to D per second is

100 108 10-15
X X
47 60 10

=

35 × 10-11 electromagnetic units.

The number of ions of either sign produced in D by the radiation from the emanation is therefore

Each particle of emanation is therefore producing ions in the surrounding gas at the rate of

We see, therefore, that as the emanation, when admitted into D, did not produce a deflexion of more than 1 scaledivision, it must either be uncharged, or, if charged, each particle of the emanation must give out radiation sufficient to produce at least 1400 ions per second. If the radiation from each particle were less than this, then the number required to give the observed ionization would be greater than what would produce 1 scale-division of a deflexion. This number is calculated on the assumption that the charge on the emanation is the same as the charge on the gaseous ion; it is not probable that it is less than this, if charged at all; and if it is greater, the number 1400 would be correspondingly greater.

may

It is, however, quite likely that each emanation particle be capable of producing ions in the vessel D at the rate of 1400 per second. For this reason the test was pushed a step further.

A me sensitive electricnter was bel and the quantity member borrasel An electrometer of the Dolezalek me was employed giving a dedexion equal to 400 male divisions per volt difference of potential between De quadrants With this sensitives the capacity of the Maceter ani electrostatic units, or

This detrometer was zei to detect the charge on the nanation, and the ionization in the vessel D. after the entation is almined, was measured by the electrometer previously pi. The mill detexion produced when air free from emanation was a imited into D' was made as small as possible before the sensitive electrometer was used, and it was finally got rid of to such an extent that the deflexion was never greater than 10 divisions, varying in diferent experiments between 2 or 3 and 10 divisions, and being always in the same direction.

We shall give numbers observed in one experiment, using the sensitive apparatus.

The deflexion on the Dolezalek was 10 divisions when the emanation was admitted. The other electrometer was then used to measure the ionization, and gave 100 divisions in 77 seconds, with a capacity of 5 mierofarad joined to it, the sensitiveness being the same as before, 60 divisions for 1 voit difference between its quadrants.

From this experiment and several similar ones we are safe in saying that in this case not more than 4 divisions of a deflexion are produced by the emanation. It is difficult to be certain of a smaller deflexion, the spot of light not being So steady as with a less sensitive instrument.

If we make a calculation of the same nature as before, we find that either the emanation is uncharged or else each emanation particle must be producing by its radiation at least 12.000 ions per second.

Even this radiation might be looked upon as quite possible, so that the question whether the emanation is charged or not would not be settled. We have, however, good reasons for believing that only a small fraction of the total emanation particles are at any instant acting as centres of radiation and ionization. The ionizing power of emanation contained in a closed vessel falls off with time in a geometrical progression, -howing that the rate of decay of the ionizing power is proportional to the ionizing power at every instant, a result which readily admits of the interpretation that the radiation arises from the emanation particles undergoing some change,

and that the number changing at any instant is proportional to the total number present.

The ionizing power I may, from experiment (Rutherford, Phil. Mag. April 1903), be represented by

I = LE-,

where is a constant and t the time measured from the instant when I = Io.

we see that is the fraction of the total emanation that undergoes change or emits radiation in one second. And we know (Rutherford, Phil. Mag. April 1903) that I falls to half its value in about four days, so that A is approximately equal to

2 × 10-6.

If, therefore, we accept the theory that the emanation undergoes a further change and that each particle acts as a centre of radiation and ionization only when undergoing change, and this is the only theory that seems to fit in with experiment, we see that the number calculated above, giving the minimum ionization that must be produced by each emanation particle in one second, assuming it to be charged, would have to be multiplied by the factor. 10o.

Multiplying 12000 by. 10°, we get 6 × 109 as the minimum number of ions produced in one second by each emanation particle when its turn comes to disintegrate, assuming that it is charged. This number is not a possible one for several reasons. Rutherford (Phil. Mag. May 1903) gives 105 as the probable number of ions produced by each a ray before it is absorbed by the gas. The ionization is chiefly due to a rays, so that to produce the above ionization each emanation particle would require to emit

The mass of the a particle being of the same order as that of the hydrogen atom, and the emanation having been produced by a disintegration of the radium atom, each emanation particle could not possibly emit more than about 200 a rays.

We can, therefore, finally conclude that the emanation not charged.

This fact that the emanation is uncharged-has an important bearing on our conception of the manner in which

The

the radium atom breaks up. The radium atom certainly gives off positively charged particles-the a rays. emanation particles cannot be what remains of the atom after the emission of one or more a rays, because, in that case, it would be negatively charged. The atom must have parted with an equal negative charge, either by the emission of negative particles or in some other way.

XLIII. The Comparison of Capacities in Electrical Work; an Application of Radioactive Substances. By J. A. MCCLELLAND, M.A., Professor of Experimental Physics, University College, Dublin*.

HERE are many methods by which two capacities may be compared, and which are fully described in textbooks of Physics.

When only approximate results are required, we have several methods to choose from, any of which will give a fair result; but the problem is by no means so simple when an accurate determination is required, especially if we are dealing with a very small capacity. That better methods of dealing with the determination of capacities, especially small capacities, are still required, may be judged from the fact that two papers have recently appeared on the subject, one by Professor Fleming and Mr. Clinton in the Phil. Mag., May 1903, and the other by Professor Stroud and Mr. Oates in the Phil. Mag., December 1903.

Those two papers may be taken as affording examples of the difficulty of obtaining accurate results in this work, both methods necessitating somewhat elaborate apparatus, and involving considerable experimental difficulties.

My object in this paper is to describe a method at once simple and accurate, and suitable for the determination of capacities of any magnitude down to a few micro-microfarads, or even less. The method is based on the fact that the ionization current that can be obtained by the use of a radioactive substance like uranium is extremely constant, and can be made so small that the time taken to charge a condenser by it can be accurately measured. This small constant current is used first to charge one condenser to a given potential; and then a second condenser is charged to the same potential, and the time taken in the two cases observed, so that we get

* From an advance proof of the Proceedings of the Royal Dublin Society, vol. x. part ii. p. 167, communicated by the Author.

the ratio of two capacities by simply observing two intervals of time.

The method will probably have occurred to any one who has been using radioactive substances; but as many workers have occasion to compare capacities accurately who are not using radioactive substances, I have thought it advisable to make a few experiments showing the accuracy of the method, and showing also how small a capacity can be detected and measured by it.

To use the method it is not necessary to have a supply of radium, as the title of the paper might suggest; uranium is even better in some respects, and uranium is to be found in every laboratory.

Description of Apparatus and Method of Working.

A and B are two insulated metal plates, one of which, B, can be joined to one terminal of a battery of small storage-cells, the other terminal of which is to earth. The battery may consist of 100 or more small test-tube cells, so that B can be kept at 200 volts or higher.

A few grammes of, say, uranium nitrate are spread on a sheet of paper, and placed on the plate A. The radiation from the uranium ionizes the air between A and B; and so A gradually rises in potential if insulated, supposing B to be positive. As is well known, the ionization current thus obtained between two plates increases at first as the potentialdifference between the plates increases; but when this potential-difference is made sufficiently great, the current attains a maximum, and does not further increase for further increase of potential-difference between the plates. If then B is kept at a sufficiently high potential, small changes in this potential, due to the potential of the battery falling, will produce no effect; and, again, in making an observation, the potential of