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*.

I

[Plate XIV.]

of

Na previous paper† some experiments were described dealing with the rate of decay and penetrating power 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 * Communicated by Prof. F. Rutherford, F.R.S. Rutherford and Allan, Phil. Mag. Dec. 1902.