The Washington herald. [volume] (Washington, D.C.) 1906-1939, 1922-06-18
Microphons

quency from entering the modula-
tor and power supply of circuits.
These choke coils are of air core
Construction and are about 5 MILL
henrys inductance each. They
thus offer a high impedance to the
radio frequency but negligible i'm
pedance to the audio frequency.
Meditation Meter Developed.
In order to indicate the amount
of meditation. a sO-called modula-
ton meter has been developed.
This consists of a current trans
| former, the primary of which is
connected in series with the II. G.
/ supply to the oscillator tube plates
and the secondary of which is con-
nected to a thermo -ammeter. The
transformer ratio is made such that
a variation in the n. c. current
from o to twice its normal value
gives full scale detection on the
meter. An air gap is provided in
the transformer core to prevent
saturation due to the ID. c. COM
/ ponet OF the plate current. The
meter has a current scale marked
| from to 100 per cent meditation.
When the announcer is speaking
into the transmitter, the modula-
tion meter averages about 40 per
cent with a maximum between 10
and so per cent. Piano solos aver
age about 80 per cent. violin solos
| 20 to 80 per cent. and vocal num-
bers 40 to so per cent with a maxi
mum of 100 per cent OF course
the meditation meter indicates only
the average volume of sound.
While the meter may read only 80
per cent in case of piano music the
individual notes at the instant of
striking may reach so to 80 per
cent.
Six 1e0-Foot Wires.
Allowing for the kind Of sound
being transmitted, that is. piano,
| speaking voice, solo. etc, the modu-
| lation meter provides a convenient
means of finding the correct dis
| lance to place the artist from the
| pickup transmitter and accounts
/ TO d Large extent for the uniform
volume of sound received from
| KDKA. Referring to the Illustra-
| ton OF the transmitting set above
| the instruments at the transmitter
panel are. from left to right, fla
| nent volt meter. ground current
| meter, and plate volt meter The
I antenna current meter Is mounted
/ o7 he wall with the series con-
\ Genser arC discharge resistance and
| is not shown in the photograph.
The antenna at KDKA consists
! Of six wires, 100 feet in length on
/ 20-foOt spreaders. A counterpoise
which is a duplicate of the antenna
| in construction. is placed 10 feet
| beneath the antenna. This brings
| the counter poise about Is feet be-
| low the transmitting set. The down
/ lead from the antenna and the coun-
| terpoise lead are made up of eight
| strands of No. II copper wire equal
/ ly spaced around l Y Inch diameter
wooden spacers. The natural period
Of this aerial system is approx
mately 412 meters. A series con-
denser of COOS MID. capacity is used
/ in series with antenna and sufficient
/ loading inductance added to obtain
the desired wave length of $50
meters. A series condenser Is
shunted by the radio frequently
choke coils of 10 millihenrys Undue
lance, to drain off any static charge
that might accumulate on the an
tenna when insulated from the
| ground by the series condenser.
OF Special Design.
The high frequency resistance of
| the antenna system at $50 meters
wave length is approximately II
| ohms. large percentage of which