the Transmitting Antenna in Wireless Telegraphy: 125 
revolution, and the field to vary in such a way that the electric 
lines of force ended perpendicular to its surface. Sarasin and 
de la Rive * and others kad compared the oscillations about a 
wire to those in an open pipe; but, as Abraham remarks, 
though the relations are essentially similar, the analogy must 
not be pushed too far. In the pipe the radiation is from 
within outwards, and is greatest in the direction of the axis ; 
while in the electromagnetic case the radiation is from 
without inwards, being limited by the surface of the wire, and 
on account of the transversality of the vibrations there is no 
radiation along the axis. Moreover, in the air-vibrations there 
is a displacement of the entire system of nodes and loops 
towards the open end, while, with the electrical oscillations, 
to a first approximation, there is no such displacement. On 
a closer examination, however, there is found to be a dis- 
placement of this kind, variable with the frequency. The 
phase of the advancing waves alters in a discontinuous manner, 
somewhat as in the vibrations of a plucked string fT. 
When two wires are used, as in Lecher’s arrangement, the 
radiation in the direction of the axis does not vanish, and the 
analogy to the open pipe is more marked. There is then a 
decided displacement of the nodes and loops, well exhibited in 
an investigation by de Forest f. 
The best acoustical analogy toa wire connected at one end 
to earth or to a large capacity and free at the other, seems to 
be a closed pipe, gas-pressure in the pipe corresponding to 
potential or charge in the case of the wire. Here there is 
a displacement of the nodes and loops, but it is very small, 
and only the odd harmonics are present in the two cases. 
Of course a rod clamped at one end is similar to the closed 
ipe. 
iakcland and Sarasin § in their investigation of the field 
about a free-ending wire explored with a circular resonator 
and found the first node distant from the end by one-half 
the circumference of the resonator (a result similar to that 
obtained by Sarasin and de la Rive in their investigation on 
two parallel wires, and ascribed by them to the geometrical 
form of the resonator), and other nodes regularly spaced 
along the wire at intervals equal to twice the diameter of the 
resonator. The form of the nodal surfaces in the space 
* E, Sarasin and L. de la Rive, Archives des Sciences Physiques et 
Naturelles, Genéve, xxiii. p. 118 (1890). 
+ Helmholtz, ‘Sensations of Tone’, p. 54; Rayleigh, ‘Theory of Sound,’ 
art. 146. 
¢ L. de Forest, Am. Jour. Sci. viii. p. 58 (1899). 
§ K. Birkeland and E. Sarasin, Comptes Rendus, cxvii. p. 618 (1893). 
