RADIOTELEGRAPH Y—FLEMING. TS 
change which in turn enables it to create, increase, or diminish a local 
current produced by a local battery and so affect a telephone or tele- 
graphic relay. One kind of change the oscillations can produce in 
certain devices is a change in their electric resistance, which in turn 
is caused to increase or diminish a current through a telephone or 
telegraphic relay generated by a local battery. To this type belong 
the well-known coherers of Branly, Lodge, and Marconi, which re- 
quire tapping or rotating to bring them back continually to a condi- 
tion of sensitiveness. Coherers, however, have been devised which 
require no tapping. Thus it has been found by Mr. L. H. Walter that 
if a short length of very fine tantalum wire is dipped into mercury 
there is a very imperfect contact between the mercury and tantalum 
for low electromotive forces. This may perhaps arise from the fact 
that tantalum, like iron, is not wetted by mercury. If, however, 
feeble electric oscillations act between the mercury and tantalum, the 
contact is improved whilst they last. If, then, the terminals of a cir- 
cuit containing a telephone in series 
with a shunted voltaic cell are con- 
nected to the mercury and tantalum, 
respectively, and if damped or inter- 
mittent trains of electric waves fall on 
an antenna and excite oscillations 
which are allowed to act on the mer- 
cury tantalum junction, then at each 
train the resistance of the contact falls, 
the local cell sends current through the 
telephone and produces a short sound, Fic. 15.—Walter’s tantalum de- 
and if the trains come frequently FECHOE: 
enough this sound is repeated and will be heard as a continuous 
noise in the telephone (see fig. 15). This sound can be cut up into 
dot and dash signals by a key in the sending instrument. If the 
transmitter is sending persistent oscillations, then some form of 
interrupter has to be inserted in the receiving circuit to enable us 
to receive a continuous sound in the telephone which can be re- 
solved into Morse dot and dash signals by the key in the trans- 
mitter. The operator usually wears on his head a double telephone, 
and listens to these long and short sounds in the telephone and writes 
down each letter or word as he hears it. The reception of signals in 
modern radiotelegraphy is most usually effected by ear, by means of 
some type of oscillation detector capable of actuating a telephone. It 
is important then to notice that, to obtain the highest sensitiveness 
when using the telephonic method of reception, the spark frequency 
or number of oscillation trains or the number of interruptions of the 
persistent train per second must take place at such a rate that it 
agrees with the natural time period of the diaphragm of the telephone 
