§ 10. Velocity of Propagation of Electromagnetic Effects through Air.—The experiments of Sarasin and De la Rive already described (see § 5) have shown that, as theory requires, the velocity of propagation of electric effects through air is the same as along wires. The same result had been arrived at by J.J. Thomson, although from the method he used greater differences between the velocities might have escaped detection than was possible by Sarasin and De la Rive’s method. The velocity of waves along wires has been directly determined by Blondlot by two different methods. In the first the detector consisted of two parallel plates about 6 cm. in diameter placed a fraction of a millimetre apart, and forming a condenser whose capacity C was determined in electromagnetic measure by Maxwell’s method. The plates were connected by a rectangular circuit whose self-induction L was calculated from the dimensions of the rectangle and the size of the wire. The time of vibration T is equal to 2π√(LC). (The wave length corresponding to this time is long compared with the length of the circuit, so that the use of this formula is legitimate.) This detector is placed between two parallel wires, and the waves produced by the exciter are reflected from a movable bridge. When this bridge is placed just beyond the detector vigorous sparks are observed, but as the bridge is pushed away a place is reached where the sparks disappear; this place is distance 2/λ from the detector, when λ is the wave length of the vibration given out by the detector. The sparks again disappear when the distance of the bridge from the detector is 3λ/4. Thus by measuring the distance between two consecutive positions of the bridge at which the sparks disappear λ can be determined, and v, the velocity of propagation, is equal to λ/T. As the means of a number of experiments Blondlot found v to be 3.02 × 1010 cm./sec., which, within the errors of experiment, is equal to 3 × 1010 cm./sec., the velocity of light. A second method used by Blondlot, and one which does not involve the calculation of the period, is as follows:—A and A′ (fig. 10) are two equal Leyden jars coated inside and outside with tin-foil. The outer coatings form two separate rings a, a1; a′, a′1, and the inner coatings are connected with the poles of the induction coil by means of the metal pieces b, b′. The sharply pointed conductors p and p′, the points of which are about ½ mm. apart, are connected with the rings of the tin-foil a and a′, and two long copper wires pca1, p′c′a′1, 1029 cm. long, connect these points with the other rings a1, a1′. The rings aa′, a1a1′, are connected by wet strings so as to charge up the jars. When a spark passes between b and b′, a spark at once passes between pp′, and this is followed by another spark when the waves travelling by the paths a1cp, a′1c′p′ reach p and p′. The time between the passage of these sparks, which is the time taken by the waves to travel 1029 cm., was observed by means of a rotating mirror, and the velocity measured in 15 experiments varied between 2.92 × 1010 and 3.03 × 1010 cm./sec., thus agreeing well with that deduced by the preceding method. Other determinations of the velocity of electromagnetic propagation have been made by Lodge and Glazebrook, and by Saunders.
On Maxwell’s electromagnetic theory the velocity of propagation of electromagnetic disturbances should equal the velocity of light, and also the ratio of the electromagnetic unit of electricity to the electrostatic unit. A large number of determinations of this ratio have been made:—
| Observer. | Date. | Ratio 1010 ×. |
| Klemenčič | 1884 | 3.019 cm./sec. |
| Himstedt | 1888 | 3.009 cm./sec. |
| Rowland | 1889 | 2.9815 cm./sec. |
| Rosa | 1889 | 2.9993 cm./sec. |
| J.J. Thomson and Searle | 1890 | 2.9955 cm./sec. |
| Webster | 1891 | 2.987 cm./sec. |
| Pellat | 1891 | 3.009 cm./sec. |
| Abraham | 1892 | 2.992 cm./sec. |
| Hurmuzescu | 1895 | 3.002 cm./sec. |
| Rosa | 1908 | 2.9963 cm./sec. |
The mean of these determinations is 3.001 × 1010 cm./sec., while the mean of the last five determinations of the velocity of light in air is given by Himstedt as 3.002 × 1010 cm./sec. From these experiments we conclude that the velocity of propagation of an electromagnetic disturbance is equal to the velocity of light, and to the velocity required by Maxwell’s theory.
In experimenting with electromagnetic waves it is in general more difficult to measure the period of the oscillations than their wave length. Rutherford used a method by which the period of the vibration can easily be determined; it is based upon the theory of the distribution of alternating currents in two circuits ACB, ADB in parallel. If A and B are respectively the maximum currents in the circuits ACB, ADB, then
| A | = √ | S² + (N − M)²p² |
| B | R² + (L − M)²p² |
when R and S are the resistances, L and N the coefficients of self-induction of the circuits ACB, ADB respectively, M the coefficient of mutual induction between the circuits, and p the frequency of the currents. Rutherford detectors were placed in the two circuits, and the circuits adjusted until they showed that A = B; when this is the case
| p² = | R² − S² | . |
| N² − L² − 2M (N − L) |
If we make one of the circuits, ADB, consist of a short length of a high liquid resistance, so that S is large and N small, and the other circuit ACB of a low metallic resistance bent to have considerable self-induction, the preceding equation becomes approximately p = S/L, so that when S and L are known p is readily determined.
(J. J. T.)