III. Conclusions.

The following conclusions seem to follow from my experiments :

1. In the simple Marconi method and the method of direct excitation, when the antenna is joined to earth, the effect is similar to using a wire the same as the antenna to balance it; that is, considered from an optical point of view, the earth acts as a plane mirror.

2. In these conditions the chief oscillation is the fundamental of the antenna, with wave-length four times its length. The condenser circuit in the method of direct excitation impresses its wave-length on the antenna, but its oscillations are not nearly so intense as those proper to the antenna itself. Thus the manner of oscillation is essentially the same in the two methods, but the latter is more regular and powerful than the former.

3. In the inductive method of excitation, on the other hand, the prominent feature of the oscillations is that one due to the condenser circuit. With antennæ of different lengths there is little change in this oscillation, the curve indicating it being decided and definite; but only onequarter of its wave-length is shown. This may be due to the great losses from radiation by the wire. The fundamental proper to the antenna is also present, but it is not nearly so intense as in either of the other two systems.

4. The effective length of the antenna for proper resonance, therefore, is one quarter wave-length, not a higher multiple. 5. When inductance is inserted between the condenser circuit and the earth the fundamental oscillation is not so regular or intense, other oscillations (overtones) being superposed.

6. For the production of oscillations by the direct method a small capacity cannot satisfactorily balance the antenna; in the inductive method, however, a capacity acts like an earth-connexion or a similar wire.

IV. Continuation of Former Investigation.

In the previous experiments with Hertzian plate oscillators of various sizes and with wires ranging in length from 300 to 860 cms., there was usually one "chief" minimum of potential-variation between 100 and 200 cms. from the free end, and always a marked one about 10 or 15 cms. from the other end of the wire. It was hoped that by employing longer wires the phenomena of standing waves would be much better exhibited, and that several "chief" minima would be shown. Such, however, has not proved to be the case. Wires 2050 and 4090 cms. long were carefully explored, the action on the wire being produced by means of an oscillator having plates 40 cms. square and the straight connexion between 60 cms. long, but the only unmistakable minimum was approximately 150 cms. from the free end, the same as was perfectly formed with wires from 300 cms. upwards.

Some evidence was obtained as to the cause of the marked minimum near the other end of the wire. It was due to the direct action of the oscillator on the detector. As described in the other paper, an attempt had been made to allow for this direct action by taking the reading when the wire was in place and also when it was removed, and then subtracting the latter from the former. This assumes that the two effects are quite independent, but such seems hardly to be the case. In the former experiments the detector lay in a horizontal plane on the top of a carriage which was moved along the wire. Thus the detector's length was parallel to that of the oscillator, though the little wing was perpendicular to it. As described in Section II. of the present paper, the detector was now hung in a vertical plane from the wire, so that its length was perpendicular to the axis of the oscillator. With this arrangement the minimum disappeared, thus showing that it had been produced by the direct action of the oscillator on the detector.

University of Toronto,
Toronto, Canada.

XIV. Radioactivity of the Atmosphere. By S. J. ALLAN, M.Sc., Demonstrator in Physics, McGill University *.

[Plate XIV.]

IN a previous paper† some experiments were described dealing with the rate of decay and penetrating power of the excited radioactivity obtained from the atmosphere on a negatively-charged wire. It was found that the activity decayed according to an exponential law with the time, falling to half its value in 45 minutes. Its penetrating power was slightly greater than that of the excited activity from radium or thorium. Its absorption by solids followed an exponential law with the thickness, and the radiation was cut down to half its value by 001 cm. of aluminium.

The amount of excited activity that could be obtained from the air at any time was found to be strongly influenced by weather conditions. A cold, clear, windy day gave the largest amount and a warm dull day the least.

In the experiments described in the present paper the radioactivity was obtained from a closed room which gave a constant amount from day to day.

The electrical method of measuring the radiations has been used throughout the experiments. The electrometer employed was of the ordinary Thomson quadrant type, fitted with a needle of light construction, which was kept connected to one pole of a battery of 300 volts. A reflecting mirror and a millimetre-scale indicated the movement of the needle. Each scale-division corresponded to 예금이 of a volt P.D. of the quadrants. A small quantity of uranium served to standardize the readings of the instrument.

Increase of Excited Activity with Time.

It has been shown in a previous paper * that the excited activity derived from the air decays according to an exponential law, the rate of decay being given by the equation

where I is the excited activity at any time t, I, the maximum value, and a a constant. If the excited activity produced on a negatively-charged wire is due to a constant supply of positive radioactive carriers whose activity decays according to the above equation, then the intensity of the activity I after an exposure for a time t will be given by

I = I (1-6-),

where I is the maximum value and the same constant as before.

The following experiment was made to verify this view :About 60 feet of copper wire was suspended in a large attic and kept charged to a constant negative potential of about 20,000 volts, by means of a Wimshurst machine driven by an electric motor. In parallel with the charged wire was arranged an adjustable spark-gap to regulate the constancy of the potential of the wire. After a certain time of exposure the wire was taken down and wound lengthwise on an iron frame. This frame was placed inside a cylindrical vessel of zinc and connected to one pair of quadrants of the electrometer, the other pair being earthed.

The zinc cylinder was connected to one pole of the battery and the other pole was to earth. Between the iron frame and the cylinder was arranged a guard-ring, to prevent any leak around the ends of the electrodes. The rate of movement of the electrometer-needle was taken as a measure of the amount of excited activity present. The wires were always tested five minutes after removal from the attic..

The results obtained from these experiments are given in the following Table, the second column giving the amount of excited activity on the wire five minutes after it was taken down.

The results are shown graphically in fig. 1 (Pl. XIV.).

From an examination of these results it will be seen that the excited activity increases with time according to the equation given above, rising to half value in about 60 minutes. It has been shown that the excited activity on a charged wire decays to half value in 45 to 48 minutes. The results are thus only approximately in agreement with theory, since it is difficult under the experimental conditions to obtain more than rough results.

Rates of Decay under various Conditions.

In the previous paper it was shown that the excited activity on a charged copper wire always had the same rate of decay, wherever and whenever produced. It was, however, deemed advisable to examine the rate of decay of the excited activity produced under as many different conditions as possible.

Iron and lead wires were tried and each gave the same rate of decay as the copper wire. Experiments were then made to see if by transferring the excited activity from the copper wire to such substances as leather and felt, by means of ammonia, any difference could be observed in the rate of decay. For this purpose about 150 feet of copper wire was suspended in the attic and kept charged for about three hours a constant negative potential of 20,000 volts. A piece of leather, about 7 cms. square, moistened with ammonia, was then rubbed over the wire, re an and by taking care to rub a fresh