The Hocking sentinel. [volume] (Logan, Ohio) 1871-1906, 1895-05-30
A COMPARISON BETWEEN ELEC.
TRICAL ENERGY AND WATER
POWER.

l.
POWER.
An Excerpt From Il Lecture Before Stu
dents of the School of Agriculture of Ohio
state University. Ivy F. c. Caldwell, Assist
ant Professor of Electrical Engineering.
Fhere is much in the working of
electrical energy that is commonly
considered mysterious, which a few
comparisons may serve to make clearer
Suppose that we have pump run by a
water-wheel or engine, and that we
connect to it pipe running to point
at distance and back again. Fill the
pipe with water and set the pump go
ing; there will result circulation of
tle water. to produce which a certain
amount of energy must be expended at
the pump, and the same amount dissi.
pated in heat in the system. Further,
the amount of water flowing through
the pipe will depend on two things,
the pressure produced by the pump
and the resistance offered by the pipe
to the flow. Now, this is very much
like what we have with a dynamo
electric machine and circuit of wire;
here again we have an expenditure of
power at the dynamo and it's conver.
sion into heat in the circuit, and here
also the strength of the current is de-
pendertupon pressure or relectromotive.
force as it is called, and the resistance.
With the water pipe the resistance to
the flow will increase with the length
and decrease with the increasing cross
section. and the same holds with the
Wire. The resistance also depends
rpon the character OF the internal sur
face Of the pipe. and similarly upon
the material of the wire; so copper
offers only about one-seventh the re-
sistance of iron. In the case of the
water, we could measure the pressure
in pounds per square inch or inches Of
mercury, or feet of water, and the
amount flowing in gallons or pounds
per minute. In electricity we have cor
responding units, the volt for pressure
and the ampere for current flowing.
To give an idea of the values of these
important units, a zinc-copper cell
gives about one volt; incandescent or
glow lamps run at so or 100 volts,
about; street cars generally about 500;
arc lamps take 80 volts for each lamp, |
SO for 90 lamps 1000 volts Again,
most incandescent lamps take about |
one-half ampere, are lights 10 am
peres. One-twcntieth ampere is
very uncomfortab if it pass through |
the human body. There is |
also a unit for resistance, called the !
ohm, and there always exists this very
convenient relation, viz, amperes=
voltsI-ohm or current-pressure ::
resistance, so if we have 100 volts and |
so ohms we have 10G+5O=2 amperes. |
10 return again to our water system;
suppose that we put a waterwheel or
turbine in at distance from our pump |
and belt it to a lathe or other machine;
we can start the pump, run the turbine
and SO the lathe, and thus transmit |
the power of the engine through the
water system. So we can put motor
into our electrical circuit aud drive it.
If with our water we want to transmit
n given amount of power, say one
horsepower, or its equivalent, $4,000
foot-pounds per minute, we can evil
IDENTIFY do it with 4 large pipe and a
slow flow of water, or we can use
small pipe and high rate of flow or
pressure. thus 1,OO0 pounds of water
and 33 feet pressure would do, or SO
pounds of water and 1000 feet pres.
sure, or any two factors that gave 3d.
00o. So with electricity, we may use
high pressure and small current, or vice
versa, and as with water the power
is feet pressure times per minute flow.
so in electricity is it volts times am
peres, for which we use the still
shorter expression watts; this watt is
the power of one ampere working un-
der n pressure of one volt. 1t is the
1-746th part of a horsepower. So. to
transmit one horsepower electrically
we can use TAG amperes at 10 volts, OF
7.46 amperes at 100 volts, or one and
one-half amperes at 500 volts, etc.
Since no machine has a perfect efficien.
cy, we must take that into account in
determining how much current will be
n necessary to furnish given power.
Thus ten-horse-power motor, with sc
per cent. efficiency, would require
49,. .s0@1S.S amperes.
Let us look at one more analogy. A
man might stand up against siX-inch
stream of water if it had head of but
q few inches without injury, while
inch stream under 1000 feet
pressure might 60 through him. Sc
with electricity, a man can take hold
Of the two terminals of one-hundred
volt machine with his two hands
and hardly feel discomfort, but if he
yets 500 volts, enough current will pass
through him, will be very painful,
and if long enough contin.
tinued might injure him. One thou.
sand volts or more, if taken for a per-
ceptible time is generally fatal To
receive a shock, the body must always
be so placed as to form part of cir
,uit between two points where diff
terence of pressure exists. Where one
side of system is grounded as in rail-
ways, shock may be received by :
standing on the earth and touching
.he overhead conductor. Although
his shock is not dangerous to man if
,nly taken for a moment, it is often IA
al to horses which are more sensitive
n this respect. This fatality could of
en be avoided without danger by
handling the wire with a piece of
10th or paper or stick, and thus re-
moving it from the animal or holding ,
t in contact with the rail, when it will
De harmless.
A