that it does not cut out some of the high notes which are necessary to
give the sound its naturalness. You will also have to make sure that
your apparatus does not distort, that is, does not receive and reproduce
some notes or "voice frequencies" more efficiently than it does some
others which are equally necessary. For that reason when you buy a
transformer or a telephone receiver it is well to ask for a
characteristic curve of the apparatus which will show how the action
varies as the frequency of the current is varied. The action or response
should, of course, be practically the same at all the frequencies within
the necessary part of the voice range.

[Footnote 7: Cf. Chap. VI of "The Realities of Modern Science."]

[Footnote 8: My thanks are due to Professor D. C. Miller and to the
Macmillan Company for permission to reproduce Figs. 79 to 83 inclusive
from that interesting book, "The Science of Musical Sounds."]

LETTER 17

GRID BATTERIES AND GRID CONDENSERS FOR DETECTORS

DEAR SON:

You remember the audion characteristics which I used in Figs. 55, 56 and
57 of Letter 14 to show you how an incoming signal will affect the
current in the plate circuit. Look again at these figures and you will
see that these characteristics all had the same general shape but that
they differed in their positions with reference to the "main streets" of
"zero volts" on the grid and "zero mil-amperes" in the plate circuit.
Changing the voltage of the B-battery in the plate circuit changed the
position of the characteristic. We might say that changing the B-battery
shifted the curve with reference to the axis of zero volts on the grid.

[Illustration: Fig 56]

[Illustration: Fig 63]

In the case of the three characteristics which we are discussing the
shift was made by changing the B-battery. Increasing B-voltage shifts
characteristic to the left. It is possible, however, to produce such a
shift by using a C-battery, that is, a battery in the grid circuit,
which makes the grid permanently negative (or positive, depending upon
how it is connected). This battery either helps or hinders the plate
battery, and because of the strategic position of the grid right near
the filament one volt applied to the grid produces as large an effect as
would several volts in the plate battery. Usually, therefore, we arrange
to shift the characteristic by using a C-battery.

[Illustration: Fig 85]

Suppose for example that we had an audi