436 
INC al OMe ah 
[SEPTEMBER 7, 1899 
LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for opinions ex- 
pressed by hts correspondents. Netther can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice ts taken of anonymous communications. } 
A Hertz Wave Receiver. 
DURING a visit to Adelaide in December last year, I was 
asked to suggest some sort of apparatus whereby Hertz wave 
disturbances might be observed. The result may perhaps be of 
some interest at the present time. The spark at the oscillator 
was small, as only a small induction coil was available. From 
previous experiments made by me, I had discovered that in 
order to obtain great sensitiveness the distance between the 
poles of a Branly detector should be as small as possible ; also 
the amount of current employed should be very small. After 
many experiments, I devised an apparatusin which only about 
1/16 of a milliampere gave excellent results. The apparatus was 
simple and easily constructed. In Fig. 1, MN are the poles of 
the permanent magnet of a D’Arsonval galvanometer ; kK, its 
coil having a resistance of 500 ohms; H, the internal soft iron 
A 
Fey 7a, 
fixed core ; LL, the flat suspension wires ; 1, a rod of ebonite to 
the side of which a minute Branly receiver is attached. This 
forms a part of the galvanometer circuit, and moves with the 
coil through its angle of displacement, the ends aB of the. 
suspension wires being fixed to the frame (not shown), 
The circuit included one small dry cell, 0, and a_non- 
inductive resistance of 16,000 ohms. In order that the Branly 
receiver C, after being made a conductor by the influence 
of a Hertz wave, may be restored to its condition of high 
resistance, it is brought up against a point F, when de- 
flected (a side view of this is shown in Fig. 2). This 
point is kept in a state of vibration by means of a jet of water, 
thus, D Eisa tube furnished with an elastic disc, to which a 
projection, F, is attached ; a jet is so fixed at E that its dis- 
charge impinges on the centre of the disc, the jet is fixed to the 
tube by a bar, QQ, the discharge is affected by the tube asa 
resonator, and hence F vibrates. The suspended coil, k, is fur- 
nished with a mirror and a pointer, whereby its movements are 
easily seen. This form of decohering instrument was used to 
avoid the evil effects due to electromagnetic vibrators, which act 
on a receiver if very sensitive. The induction wings were con- 
NO. 1558, VOL. 60] 
nected to the apparatus at Aand B. On repeating the experi- 
ment at home, I found that 1/16 milliampere is by no means 
the smallest current that might be used ; all that is required is a 
current sufficient to move the suspended coil. I am informed 
that the current used in wireless telegraphy is usually about one 
milliampere ; it is obvious that by increasing the sensitiveness of 
the galvanometer a more sensitive coherer may be used. By 
means of a vane, S, moving in liquid the movements of the coil 
are damped. The cohering substance was an 8 per cent. alloy, 
| made in the oxyhydrogen flame, and then reduced to filings in 
the usual way ; it was found to be exceedingly sensitive, much 
more so than the mechanical mixture of filings of the two 
metals. 
I made several attempts to use a sensitive galvanometer for 
closing the circuit ; the results were unsatisfactory, the closing of 
the circuit was uncertain, and when it was closed the tendency 
to stick, due to the contacts, was a source of much trouble. It 
then occurred to me that the whole difficulty would be obviated 
by attaching the coherer to the moving axis of the galvanometer 
coil itself, for by this means the contact is entirely avoided, 
while the coherer is brought within the range of a constantly 
vibrating projection which causes immediate decoherence. A 
vibrating reed was tried as a decoherer, but abandoned owing 
to the trouble of feeding it with air under pressure. Another 
form of decohering vibrator was also tried, which consisted of 
a long tube, part of which was glass ; each end was furnished 
with a tympan, one of which was placed as F and p, Fig. 2; 
the other tympan was led into a metal box containing an 
electromagnetic vibrator, the hammer of which beat upon the 
tympan remote from the galvanometer. The electromagnet 
may thus be placed at a great distance from the apparatus, 
while its impulses are communicated through the columin of air 
in the tube. 
Since my return home, I have used the Wehnelt circuit 
breaker for producing Hertz waves with ordinary oscil- 
lators ; the effects appear to be perfect, although the space at 
my disposal, about two miles, is for the most part covered with 
houses and high buildings. F. J. JeERvis-SMITH. 
Oxford, August 26, 
Is Insusceptibility to Vaccine produced by Small-pox ? 
IF vaccination inhibits, arrests, modifies, or mitigates variola 
because it is one with variola, if the attenuated virus and local 
eruption interfere with the more virulent and generalised erup- 
tion, may not a reciprocal antagonism be expected? If the- 
minor malady interfere with the major malady, how much more 
should the major malady, within a reasonable period of years, 
confer some degree or manner of general constitutional protec- 
tion in respect of the minor, if not a modification of the local 
result of vaccination ? If I tnistake not, systematic investigation 
on this point, on the human subject, has been strangely neglected 
in England, if not elsewhere. 
CHARLES G. STUART-MENTEATH. 
23 Upper Bedford Place, W.C., August 14. 
SMALL-POX does leave behind it an insusceptibility to vaccinia. 
If the writer of the letter will refer to Dr. Monckton Copeman’s 
article on ‘‘Variola and Vaccinia, their Manifestations and 
Inter-relations in the Lower Animals: a Comparative Study ” 
(Journ. of Path..and Bact., vol. il., 1894, p. 408, e¢ seg.), 
he will find references to the protection conferred by variola 
against vaccinia. That systematic investigation has not been 
carried out is probably due to the fact that the subject has so 
little practical interest. 
From the point of view of those interested in the general 
question of immunity, this subject is well worth careful and 
systematic study. G. Sims WOODHEAD. 
THE DOVER MEETING OF THE BRITISH 
addition to pre- 
ASSOCIATION. 
iTS the arrangements described 
viously, an installation of Marconi’s system of wire- 
less telegraphy will be set up in the Maison Dieu Hall 
of the Town Hall. There will be constant communica- 
tion with the South Foreland, the East Goodwin Light- 
ship and Wimereux-Boulogne. By ‘means of this 
arrangement visitors will be kept fully informed of the 
| proceedings of the French Association at Boulogne. 
