894 
Professor Lodge, that we have in the meas- 
urement of the variable resistance of this 
instrument a means of determining the in- 
tensity of the energy falling upon it. This 
variation is being investigated both as re- 
gards the magnitude of the energy and the 
frequency of the incident waves. Now 
such electrical effects are well known. In 
1866 Mr. S. A. Varley introduced a light- 
ning protector constructed like the above 
tube, but made of boxwood and containing 
powdered carbon. It was fixed asa shunt 
to the instrument tobe protected. It acted 
well, but it was subject to this coherence, 
which rendered the cure more troublesome 
than the disease, and its use had to be 
abandoned. The same action is very com- 
mon in granulated carbon microphones like 
Hunting’s, and shaking has to be resorted 
STEEPHOLM 
@ Mareonl Experiments 
e==—=Eleotro-Magnetio Induction Experimonts 
Fia. 3. Map of locality where the experiments were 
carried out. 
to, to decohere the carbon particles to 
their normal state. M. E. Branly (1890) 
showed the effect with copper, aluminium 
and iron filings. Professor Oliver Lodge, 
who has done more than any one else in 
SCIENCE. 
[N. S. Von. VI. No. 155- 
England to illustrate and popularize the 
work of Hertz and his followers, has given 
the name ‘coherer’ to this form of appa- 
ratus. Marconi ‘decoheres’ by making 
the local current very rapidly vibrate a 
small hammer head against the glass tube, 
which it does effectually, and in doing so: 
makes such a sound that reading Morse 
characters is easy. The same current that 
decoheres can also record Morse signals on 
paper by ink. The exhausted tube has two: 
wings which, by their size, tune the re- 
ceiver to the transmitter by varying 
the capacity of the apparatus.* Choking 
coils prevent the energy escaping. The 
analogy to Professor Silvanus Thompson’s. 
wave apparatus is evident. Oscillations. 
set up in the transmitter fall upon the re- 
ceiver tuned in sympathy with it, coherence: 
follows, currents are excited and 
signals made. 
In open clear spaces within 
sight of each other nothing more 
is wanted, but when obstacles. 
intervene and great distances are 
in question height is needed; 
tall masts, kites and balloons. 
have been used. Excellent sig- 
nals have been transmitted be- 
tween Penarth and Brean Down, 
near Weston-super-Mare, across. 
the Bristol Channel, a distance 
of nearly nine miles (Fig. 3). 
[The system was here shown in 
operation. | 
Mirrors also assist and inten- 
sify the effects. They were used 
in the earlier experiments, but. 
they have been laid aside for the 
present, for they are not only expensive to. 
make, but they occupy much time in manu- 
facture. 
*The period of vibration of a circuit is given by 
the equation T = 27 V K L, so that we have simply 
to vary either the capacity K or the so-called ‘ self- 
induction ’L to tune the receiver to any frequency - 
It is simpler to vary K. 
