188 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
particular wave length of the received signal. As in the case of the 
transmitting set, these adjustments are ordinarily made before the 
plane leaves the ground, and it is usually necessary for the aviator to 
make in flight only an extremely small adjustment in the capacity to 
insure the proper tuning of the receiving set. Because of the con- 
dition of vibration existing on the airplane and the effect of this 
vibration on the vacuum tubes used in the receiving apparatus, it 
was necessary to devise a mechanical filter which would protect the 
vacuum tubes from these vibrations. The method finally adopted 
consisted of the use of sponge rubber supports for certain elements 
within the set box itself. 
The current required by the plate circuits of the receiving tubes 
and the negative grid potential for the modulator tube of the trans- 
mitting set are furnished by small dry batteries, which consist of 15 
cells of very special construction weighing less than 1 pound and 
having sufficient current capacity to operate the set a few hours a 
day for a considerable number of weeks. The most important re- 
quirement for these batteries, however, was a sufficient length of 
shelf life, inasmuch as a period of several months usually elapsed 
between the date of manufacture and the date of use. Difficulties of 
securing even a few months' shelf life for cells of such small size 
made the use of standard cells out of the question. Development of 
particular types of containers for the cells and extremely careful 
selection of materials resulted in the satisfactory solution of this 
problem. 
ANTENNA. 
The form of antenna which was adopted for the early experiments 
with the airplane wireless telephone consisted of a wire trailing 
behind the airplane in flight and connected through the apparatus 
with the frame of the airplane as a counterpoise. The length of this 
wire was ordinarily 300 feet. A reel was provided for holding this 
wire and a small weight was attached to the free end to cause it to 
unreel properly after the airplane left the ground. In order to reduce 
the attention required by the aviator in unreeling the antenna a 
special form of reel was devised with a centrifugal governor, which 
limited the unreeling speed to a value which prevented the weight 
from breaking the wire at the end of the unreeling process. The wire 
which was used was a soft braided copper wire made purposely of 
low tensile strength, so that in case this wire became entangled with 
any obstruction during flight no particular strain would be put upon 
the airplane structure before the antenna wire would break. 
The subject of radiation from airplane antennae has perhaps re- 
ceived less attention in proportion to its importance than almost 
