each week. Then, having figured his sales, his profits can be
obtained by referring to the graph. See Fig. 17. This is a
simple equation; all equations may be used in a similar manner.
DR. G. E. OHM'S EQUATION
Equations are "tools" which Radio-Tricians are continually
using. You saw how easy it was to find the amount of profit by
using the equation p=s/4 if the total amount of the sales (s)
was known. It is clear then, that if any two terms of an
equation are known, the third (unknown) can be easily
calculated.
In Radio, we have one equation in particular which is
fundamental. You remember in our vacuum tube circuit we
Fig. 17
spoke of the resistance which the filament (heater) offered to
current flow. Dr. Ohm, years ago, discovered that the amount
of resistance in a circuit bore a very definite relationship to the
pressure (voltage or e.m.f.) and the amount of current flowing
(measured in amperes).
He discovered that the amount of current flowing was
always equal to the amount of the e.m.f. divided by the amount
of resistance. Being a scientific man and so a seeker after
short cuts, he simplified this statement by using letters as
symbols for e.m.f. and current, and wrote the statement in the
form of an equation--
I = E ÷ R or E / R
"I" represents current, "E" represents e.m.f. (voltage) and "R"
stands for resistance.
