A little study of this graph (Fig. 8) will show that everything
unnecessary has been ;eliminated. We know the values
we are dealing with--years and average sets per week.
All we want to see is whether the average number of sets has
increased or decreased, when there has been an increase or a
decrease and the amount of the increase or decrease. This
graph tells us the story in the simplest and dearest way the
mind of man has been able to discover.
In Radio work, especially for showing changing electrical
values, graphs are always used. It won't be long before a Radio
graph will bring to your mind very definite pictures of the
actions of various Radio devices--of what is going on inside a
Radio receiver when it is "picking music and speech out of
the air."
One particular action--the flow of electricity in the plate
circuit of a vacuum tube--is very much like the flow of water
Fig. 9
in a pump circuit. Let's go on and see how much Radio we can
learn from observing the pump in action. Fig. 9 is a drawing
of a laundry's water supply circuit showing the reservoir from
which the water is pumped to supply the laundry and a
watermeter which indicates the number of gallons flowing through
the circuit per minute. The laundry man is very much interested
in this, for the operating speed of his plant depends largely on
getting water in sufficient quantities.
We assume that the pump is of the paddle-wheel type and
that it is possible for us to count the number of revolutions it
makes per minute (its speed). Naturally the rate of the water
flow varies with the speed of the pump. Now we want to find
the relationship between the speed of the pump and the rate of
water flow. We count the number of revolutions at a certain
speed, and note the meter reading at that speed. We might
