fifteen cents each. The chief difference is that the former are
capable of giving larger current than the latter, when working against
very small resistance. For example, the former may give twenty to
twenty-five amperes on a short circuit, that is, connected directly
with the ammeter without other resistance, while the latter may give
not more than six to ten amperes under similar conditions. For most
purposes, other than igniting gasolene engines, in which dry cells
are used, an exceedingly small current is required. The electric
bell, for example, may not require more than .2 of an ampere and that
intermittently. Now it is found by experience that the dry cells which
are only capable of furnishing on short circuit six to ten amperes will
last quite as long in bell work as one which may give on short circuit
twenty to twenty-five amperes. Hence it is good economy to buy them.
"What a fine sitting room you have here! (Fig. 107, _f._) When do you
expect to fit it up?" said I. Instantly reminding myself, however, that
boys do not want a sitting room, I inquired what they intended to use
this fine, large room for. They told me that they had plans for making
a machine shop out of that. The idea had been suggested by a counter
shaft which still hung from the ceiling, and they had discovered that
the old mill wheel would still roll over if the penstock were repaired.
I replied that I would see what could be done about that sometime.
On the next day matters concerning the motor boat engaged our
attention.
X
HOW ELECTRICITY FEELS
What is more fickle and yet more fascinating than a motor boat? On the
morning after my arrival at Millville the boys wanted me to go out with
them in the motor boat on the mill pond, as our beautiful little lake
is called.
Each one took a hand at trying to start the boat, but although she had
acted perfectly well the day before, on this morning no one could get
a single explosion. The switch was closed. The gasolene was turned on.
The carburetor valves were set at the mark. The spark coils responded
with their familiar buzz. She had been primed and, when she had refused
to respond to this treatment, the pet valves were opened and the wheel
rolled over several times to sweep out the cylinders. But absolutely
nothing moved her--neither coaxing nor gibes. Suddenly some one rolled
the wheel over for the five-hundredth time and she started and behaved
well all day.
All this would not have given us the slightest aggravation if we could
only have found out what was the matter and what it was we finally did
to correct it. But this we shall probably never know, and hence we are
worshippers of the motor boat while we continue to distrust it and
complain of it.
While the boat was running one of the boys noticed that a binding
post at the end of one of the spark plugs seemed to be loose. He
inadvertently put out his hand to tighten it and received a terrific
shock. This raised the question among the boys, why one gets a shock
from some of the binding posts in the electrical equipment but not from
others. I suggested that we run in and call at the house to get my
portable measuring instrument (Fig. 110) and a little lunch, and then
go up to the upper end of the lake and take our time in examining the
electrical equipment of the boat.
[Illustration: Fig. 110]
The engine had two cylinders. There were two batteries--one for each
cylinder. Each battery consisted of five dry cells like the one
represented in Fig. 111.
"Now, why don't I feel the electricity when I touch the binding posts
of this dry cell?" inquired one of the boys as he handled one of the
cells which we had taken out. "Well, I'll give you two reasons why do
you not feel it," said I. "First, because you were touching only one
binding post at a time. You must touch both of the binding posts of
the battery cell at the same time, so that the electric current may
pass from one post to the other through your body. Second, even when
you do touch both binding posts at the same time you feel no current,
simply because you offered probably about 100,000 ohms of resistance
to the passage of the current and inasmuch as the one cell exerts only
1.5 volts of pressure, it could send only about .0000015 of an ampere
through you. This you cannot feel.
[Illustration: Fig. 111]
(1.5 volts)/(100,000 ohms) = .0000015 amperes.
"I now connect my instrument as a volt meter between the binding posts
of the cell and you see it indicates 1.5 volts, and when I connect it
for an instant as an ammeter you see it indicates twenty amperes. That
is twice as much as they use for executing criminals by electricity.
So you see if you could reduce your resistance sufficiently this one
battery cell might kill you. Some people have less resistance than
others. The resistance of the body is chiefly in the outer skin. If
one's hands are dry and his skin has been made tough and horny by hard
work, he has many times the resistance of one whose hands are moist and
whose skin is thin and tender.
"Suppose we select the tip of the tongue as the portion of the body
which will offer the least resistance and will be most sensitive to
slight electric currents. Let us then connect one dry cell with the
ammeter and place the tip of the tongue between the bare ends of the
wire at _T_ (Fig. 112).
[Illustration: Fig. 112]
"I have connected the ammeter so that it will indicate thousandths of
an ampere, and you see that the needle moves only slightly. We cannot
call it more than .001 ampere." Each boy in turn tried sending the
current through his tongue and each tried to tell how it felt. One
said it tingled, another said it felt warm, another said it tasted
sour and the other said he did not feel or taste anything. "Well," I
said, "whether you feel anything or not one-thousandth of an ampere is
passing through your tongue and you are offering fifteen hundred ohms
of resistance.
(1.5 volts)/(1500 ohms) = .001 ampere
"Your hand offers nearly seventy times as much resistance as your
tongue. Suppose we try increasing the voltage, or pressure, of our
electric current. We will connect in series the ten cells, making
a battery which you see by the volt meter gives fifteen volts of
pressure. We now find that having ten times the pressure it sends ten
times as much current as formerly through the tongue."
(15 volts)/(1500 ohms) = .01 ampere
Each one now testified that the battery sent all the current he cared
to take through his tongue. If they send one thousand times as much as
that through a criminal no wonder it kills him. It produces a twitch
when the contact is first made, afterward a decided sensation of warmth
and acid taste.
If we should increase the voltage tenfold more, say the 110-volt dynamo
current (direct current), and touch the bare conductors with our hands,
the ammeter would indicate about .001 ampere. That is, although this
current has about seventy times as much push, or voltage, as a dry
cell, no more electricity passes through the fingers than did through
the tongue in the preceding experiment with one cell. The fingers offer
so much greater resistance.
By wetting the fingers and pressing them firmly upon the bare wires,
we may make the ammeter read .01, that is, we may increase the current
tenfold by reducing the resistance to one tenth. But there is nothing
disagreeable about the feeling. If the same experiment is tried with
the 110-volt alternating current, although the quantity of current
which passes through the fingers is the same as before, the tingling
is more perceptible than in the case of the direct current. If we
join together seventy-five dry cells, giving a voltage of 112, and
press the bare wires with our wet fingers, the ammeter will indicate
.01, but there is no tingling sensation, merely a slight warmth. The
battery current, being continuous, causes no twitching of the muscles
while the contact is closed. The direct current dynamo furnishes a
slightly pulsating current. Hence, one may tell by the feeling whether
an electric current comes from a battery or a direct current dynamo.
The alternating-current dynamo gives a surging of electricity back
and forth in the wires, and this may be distinguished from the direct
current by its feeling; when, however, the number of alternations per
second is increased very greatly, one may receive through the body
considerable quantities of electricity without feeling it. With a very
high frequency current one may put himself in circuit and light a
16-candle-power lamp without any disagreeable sensation.
The outer skin is our chief insulation. If it is dry and well toughened
by work it offers a resistance of over 100,000 ohms upon gentle
contact. A wounded spot, or places like the tongue with moist, thin
skin, may offer a resistance as low as 500 ohms. If one has a pin prick
or a splinter in his hand which he cannot locate, he may hold one bare
wire of a 110-volt alternating circuit in one hand and move the other
bare wire about on the suspected region, and know when it reaches the
spot by a tingling sensation.
[Illustration: Photograph by Helen W. Cooke. Feeling Electricity]
One may touch lightly the 220-volt direct current and scarcely note any
difference between this and the 110-volt direct current, because one is
not very sensitive to the difference between .001 ampere and .002
ampere passing through his body.
(100 volts)/(100,000 ohms) = .001 ampere,
and
(200 volts)/(100,000 ohms) = .002 amperes
Physicians treat certain ailments by the use of the electric current.
For this purpose they invariably use a pulsating or alternating current
and reduce the resistance by using metal handles and wet sponges for
contact with the skin, but even so a very small amount of current
passes. The moderate twitching of the muscles seems to be the end
sought.
Men who are supposed to be killed by electric shocks often die from
other causes. A man perching upon an electric light pole, repairing
wires, may come in contact with a wire charged, say, to 2000 volts. He
may receive a shock which throws him in an unconscious condition across
another live wire which burns its way into his flesh, or he may fall to
the ground and be killed by the fall. A workman may hold a tool so as
to short circuit a current through it, making it red hot in his hands.
So many men who have been shocked into unconsciousness by high voltage
currents have recovered consciousness later that we cannot say how much
current is required to kill a man. For the execution of criminals 1800
to 2000 volts are used, and by special metal contacts ten to fourteen
amperes are forced through the body.
The first execution of a criminal by electricity was performed in Sing
Sing Prison, New York State, in 1890. There was at that time a hot
controversy among experts over the question whether death, or merely
unconsciousness, could be produced by electricity. To be on the safe
side the legislature passed a law requiring that the electrocution of a
criminal should be followed immediately by the dissection of his body.
Only six states out of forty-nine have thus far adopted that method
of capital punishment, five have abolished capital punishment, and
thirty-eight still prefer hanging to electrocution. But it should be
remembered that it is amperes, not volts, that kill. One often hears
the meaningless expression, "he received 2000 volts into his body."
The volts indicate the pressure, analogous to pounds per square inch
of water pressure. Amperes of electricity are analogous to gallons of
water. It is possible to have exceedingly high voltage of electricity
without amperes enough to do damage. When one holds his finger near to
a rapidly moving leather belt and a stream of sparks passes between the
finger and the belt, the voltage may be 50,000 or even 100,000, but
the quantity in amperes is too small to do any damage or even produce
much sensation. A similar thing is true when one produces sparks by
rubbing a cat's back, or lights the gas by a spark produced by rubbing
the feet upon a carpet. Such sparks are miniature lightning discharges.
The real lightning does damage because it furnishes quantity,
measurable in amperes, as well as extremely high volts of pressure.
At this point I was reminded by the boy who had received a shock from
the engine that morning that he had touched only one binding post.
How then had he closed a circuit through his body, and how could he
receive such a terrible shock when there were only a few battery cells
to produce the electric current. I replied that he had the distinction
of having encountered about a 5000-volt current. In the language of the
newspapers he might say, _Took 5000 volts and still live._ We must next
proceed to show how he really did close the circuit and how the spark
coil enables a battery of a few dry cells to produce exceedingly high
voltages.
XI
THE ELECTRICAL SPARKING EQUIPMENT FOR A GASOLENE ENGINE
Under the shade of a great sugar maple, with Millville Lake spread
before us, we took apart and examined the entire equipment for
producing the electric sparks to explode the mixture of gasolene and
air in the cylinders of our motor boat. The engine has two cylinders.
For each cylinder there is a separate battery and spark coil. Inasmuch
as the electrical outfit is duplicated for each cylinder it will be
necessary for us to consider the case of one cylinder only.
When this engine is running, 700 explosions per minute are produced in
each cylinder. In one-twelfth of a second the following four events
take place:
1. The cylinder is swept clear of the products of combustion formed by
the last explosion.
2. Four drops of gasolene are vaporized and mixed with one quart of air
and pushed into the cylinder by the pressure of the atmosphere.
3. This mixture is compressed by the piston in the cylinder to about
one-fifth its original volume.
4. The mixture is heated to its kindling temperature, which is above
2000 degrees. It then burns with a sudden expansion, which drives
the piston before it and pushes the crank which is concealed in the
lower end of the cylinder half-way around. The crank is attached to
the shaft, which carries the fly-wheel upon one end and the propeller
wheel upon the other end. The momentum of the moving parts--chiefly
that of the fly-wheel--suffices to accomplish the remaining half of the
revolution.
That any machine could be devised which could repeat these four events
700 times a minute was unthinkable a few years ago.
The first men who thought that a gasolene engine could be a practical
thing were considered visionaries, but now they are found to be more
practicable than steam engines. They are so efficient that they compete
with the steam engine upon its own ground, and, in addition, they
have opened up regions of usefulness which the steam engine can never
exploit. So far as we can see, they have a permanent monopoly of the
navigation of the air.
It is with the fourth event mentioned above, viz., kindling the
explosive mixture, that we are now concerned. The high temperature
required for this is obtained by forcing an electrical current against
resistance.
Five dry battery cells would very readily heat a short piece of fine
wire to a sufficiently high temperature to explode the mixture, but
it is impossible to alternately heat and cool a wire twelve times a
second. It is too slow an operation. The only other method known at
present is to imitate the lightning and force an electric current
against the resistance of the air with sufficient power to produce the
required heat. This, however, requires an extremely high voltage--at
least 5000 volts, and our battery of five cells has not more than seven
and a half volts of pressure. The interesting question then is, how
does the spark coil enable us to raise the voltage from 7 to 5000.
To help toward an understanding of the matter I took seven small wire
nails which I found in the boat--they were sixpenny finishing nails. I
then took two or three yards of No. 24 insulated magnet wire, such as
is used upon electric bells, etc. I use it more often than any other
wire, and always have some about the boat. I fastened one end of this
wire to one of the binding posts of a dry cell (Fig. 113), _a_, and
attached branches _c_ and _d_ to it. The other end, _b_, was left
free to act as a switch for closing the circuit by touching it to the
remaining binding post.
[Illustration: Fig. 113]
[Illustration: Fig. 114]
[Illustration: Fig. 115]
One boy then touched the bare ends _c_ and _d_ to the tip of his
tongue, while I touched repeatedly the binding post with _b_. There
was, of course, no sensation. We now wound a portion of the wire upon
the bundle of nails, laying on about fifty turns. (See Fig. 114.) The
tongue was now placed at _T_ and _b_ was touched a few times to the
free binding post. A very decided shock was felt, not while the end
of the wire was resting upon _b_, but at the instant of touching and
again at breaking the connection. The shock was noticeably stronger at
the instant of breaking than of making the connection. There was also
a spark formed when the connection was broken, which did not appear
before the coil was made. We next wound on more of the wire--about
fifty more turns (Fig. 115). When now connections were made and broken
at _b_ the tongue at _T_ felt a much more decided shock, and a larger
spark occurred at _b_ when the circuit was broken. Both the tongue
and the spark indicate that the voltage is creeping up very rapidly
in this series of experiments. We next connected two cells in series,
then three, four, and finally five cells in place of the one. The spark
grew larger and "fatter," as the boatmen say, with each addition of a
cell. It was not pleasant to use the tongue in the experiment after
the number of cells exceeded two. I removed the branch _d_ from the
wire _b_ and connected it to the binding post, as shown in Fig. 116.
I then removed the crystal from my watch and poured into it a little
gasolene. I rubbed the ends of _b_ and _d_ together over this, and
when they separated the spark which was produced would not light the
gasolene. We had made a coil which produced a spark that looked like a
miniature flame, but still was not hot enough to set fire to gasolene
vapour. It simply needs more iron in the core and more turns of wire
about it. Bringing the ends of the wires together and separating them
is somewhat like drawing an arc with the arc light carbons. It requires
a vastly higher voltage to make a spark jump across an air gap than it
does to lead it across thus.
[Illustration: Fig. 116]
The kind of coil we have made (only larger) is very much used in houses
as a gas-lighting coil (to be described later). It is very much used
also for exploding gasolene engines. It generally passes under the name
of the "make and break" coil. The revolving shaft of the engine is made
to push together the ends of the wire and separate them at the right
instant to make the spark for explosion. Of course this is done inside
of the engine cylinder.
That type of coil does not offer resistance enough to protect the
battery, and dry cells soon run down if used with it. The coils that
we have in this boat are somewhat different from that, the details of
which we cannot now entirely explain.
They offer enough resistance to cut the current required of the battery
down to one third what the "make and break" coil would take and at the
same time they raise the voltage so much higher that the spark will
jump across an air gap without being led across as an arc. Hence they
are called "jump spark" coils.
[Illustration: Fig. 117]
It will be remembered that when we were studying the dynamo we produced
an electric current by moving a magnet. We may now add that an electric
current may be produced by simply changing the strength of a magnetic
field. The coil that we have just made creates a magnetic field in
the region about itself whenever a current is passing through it. The
tongue at _T_ (Fig. 117) detects an extra current while the magnetic
field is being produced, or while it is dying away, or it will detect
any slight variations in the strength of the current which produces
the magnetic field. It is customary to distinguish between these two
currents. The battery current which produced the magnetic field is
called the primary current and the current which is detected by the
tongue is called the secondary current. The primary current in our
experiments had only a few volts of pressure, from one to seven. The
secondary current had many volts, as indicated by the spark. If we
rub the end of the wire _c_ across the binding post under _b_ (Fig.
117) no spark occurs. The current does not in this case go through
the coil, and no secondary current is produced. Whenever we touch the
wire _b_ to that post we have, in addition to the primary current
which has not voltage enough to produce a spark, a secondary current
flowing in the same wire at the same time and having voltage enough to
produce a spark. The primary current is continuous while the contact
is closed; the secondary current is momentary, as the tongue detects,
and is produced only while changes are being made in the strength of
the magnetic field. We will now take another piece of wire and wind
upon the coil about two hundred more turns, leaving this outer coil
wholly disconnected from the inner one, (Fig. 118). I connect _c_ and
_d_, the terminals of what we may call the secondary coil, with my
measuring instrument and I connect _a_, one of the terminals of the
primary coil, with the battery. I then rub _b_, the other primary
terminal across the free binding post of the battery. At the instant
of closing the primary circuit--that is, of building up the magnetic
field--a secondary current is induced in the secondary coil, which
lasts for only an instant, too brief a time for the needle to measure
it, although its motion indicates both the presence and the direction
of the induced current. While the primary circuit remains closed--that
is, while no change is occurring in the strength of the magnetic
field--the needle returns to zero, indicating no secondary current. But
when now the primary circuit is broken and the magnetic field loses its
strength, the needle indicates a momentary current in the secondary
coil and _in the opposite direction from what it had been at first_.
[Illustration: Fig. 118]
If, therefore, I rapidly make and break the current at _b_ I produce an
alternating current in the secondary coil. I will connect _c_ and _d_
with a miniature lamp and, resting a coarse file upon the free binding
post, I will rake the end of the wire _b_ up and down upon this file so
that, as it dances along upon the file, it will rapidly make and break
the primary circuit, and therefore rapidly change the strength of the
magnetic field. You notice that the lamp lights up moderately well. It
is being lighted by an alternating current. I move the wire a little
more slowly and you see the flicker of the alternations. According to
the label upon the lamp it requires ten volts, and our battery could
not give that. We have therefore "stepped up" the voltage as we say and
we have a veritable step-up transformer.
In this case the primary and secondary circuits are entirely separate.
It is a familiar fact that different electric currents may pass through
the same wire at the same time without apparent conflict. We send
numerous telegraph despatches through the same wire at the same time.
It is quite as easy for several pairs of persons to telephone over the
same wire at the same time as it is for those same several pairs to
carry on separate conversations in the same room at the same time, at,
say, an "afternoon tea." We may use the same wire at the same time to
carry direct and alternating currents. This fact was first discovered
in 1902 by Bedell of Cornell University.
Primary and secondary currents do not require separate primary and
secondary coils to convey them. They may or may not be connected into
one continuous coil. It is quite immaterial whether they are connected
or not so long as they are in the same magnetic field. Indeed, it seems
that the field outside of the wire may be quite as important as the
wire itself.
[Illustration: Fig. 119]