Motors Part 9
The electric current has many peculiar manifestations, the causes of some of them being known and utilized. In the use of this medium for igniting the fuel gas, many of the phases of electrical phenomena are brought into play, and it is necessary, therefore, to know something of the fundamentals of the science to enable us to apply it.
Characteristics of Electricity.--When a current pa.s.ses along a wire, it does not describe a straight path, but it moves around the conductor in the form of circles. The current is not confined wholly to the wire itself, but it extends out a certain distance from it at all points.
Magnetic Field.--Every part of a wire which is carrying a current of electricity has, surrounding it, a magnetic field, of the same character, and to all intents and purposes, of the same nature as the magnetic field at the ends of a magnet.
Elasticity.--This current has also something akin to elasticity. That is, it surges to and fro, particularly when a current is interrupted in the circuit. At the instant of breaking a current in an electric light circuit there is a momentary flash which is much brighter than the normal light, which is due to the regular flow of the current.
This is due to the surging movement, or the elastic tension, in the current. Advantage is taken of this characteristic, in making a spark.
This spark is produced at the instant that the ends of the wires are separated.
The Make and Break System.--No spark is caused by putting the two ends together, or by making the connection, but only by breaking it, hence it is termed the _make_ and _break_ method of ignition.
When the connection is broken the current tries to leap across the gap, and in doing so develops such an intense heat that the spark follows. As a result of the high temperature it is necessary to use such a material where the gap is formed that it will not be burned. For this purpose platinum, and other metals are now employed.
Voltage.--This plays an important part in ignition. Voltage is that quality which gives pressure or intensity to a current. It is the driving force, just as a head of water gives pressure to a stream of water.
High and Low Voltage.--A high tension current,--that is, one having a high voltage, will leap across a gap, whereas a low voltage must have an easy path. When the ends of a wire in a circuit are separated, air acts as a perfect insulator between them, and the slightest separation will prevent a low current from jumping across.
This is not the case with a high tension current, where it will leap across and produce the flash known as the _jump spark_.
Low Tension System.--Two distinct types of ignition have grown out of the voltage referred to, in which the _make_ and _break_ system uses the low tension, because of its simplicity in the electrical equipment.
Disadvantages of the Make and Break.--There is one serious drawback to the extended use of this system, and that is the necessity of using a moving part within the cylinder, to make and break the contact in the conductor, as it is obvious that this part of the mechanism must be placed within the compressed mixture in order to ignite it.
Amperes.--A current is also measured by amperes,--that is, the quant.i.ty flowing. A large conductor will take a greater quant.i.ty of current than a small one, just as in the case of water a large pipe will convey a greater amount of the liquid.
Resistance.--All conductors offer resistance to the flow of a current, and this is measured in _Ohms_. The best conductor is silver and the next best is copper, this latter material being used universally, owing to its comparative cheapness.
Iron is a relatively poor conductor. Resistance can be overcome to a certain extent, however, if a large conductor is used, but it is more economical to use a small conductor which has small resistance, like copper, than a heavy conductor, as iron, even though pound for pound the latter may be cheaper.
Direct Current.--There are two kinds of current, one which flows in one direction only, called the _Direct_. It is produced in a dynamo which has a pair of commutator brushes so arranged that as the armature turns and its wires move through the magnetic fields of a magnet, and have direction of the current alternate, these brushes will change the alternations so the current will travel over the working conductors in one direction only.
Primary and secondary batteries produce a direct current. These will be described in their appropriate places.
Alternating Current.--This is a natural current. All dynamos originally make this kind of current, but the commutator and brushes in the direct current machine change the output method only. The movement of this current is likened to a rapid to and fro motion, first flowing, for an instant, to one pole, and then back again, from which the term _alternating_ is derived.
While the sudden breaking in a circuit will produce a spark with either the direct or the alternating currents, the direct is usually employed for the make and break system, since batteries are used as the electrical source.
On the other hand the jump spark method employs the alternating current, because the high tension can be most effectively produced through the use of _induction coils_, which will be explained in connection with the jump spark method of ignition.
Generating Electricity.--There are two ways to produce a current for operating an ignition system, one by a primary battery, and the other by means of a magneto, a special type of dynamo, which will be fully explained in its proper place.
Primary Battery.--As we are now concerned with the make and break system, the battery type of generation, and method of wiring up the same, should first be explained.
Thus, in Fig. 34, a primary battery is shown, in which the zinc cell A has an upwardly-projecting wing B at one side, to which the conductor is attached; and within, centrally, is a carbon bar C. An electrolyte, which may be either acid or alkali, must be placed within the cell.
[Ill.u.s.tration: _Fig. 34. Dry Cell._]
Making a Dry Cell.--The zinc is the negative, and the carbon the positive electrode. The best material for the electrolyte is crushed c.o.ke, which is carbon, and dioxide of manganese is used for this purpose, and the interstices are filled with a solution of sal-ammoniac.
The top of the cell is covered with asphaltum, so as to retain the moistened material and the liquid within the cell, and thus const.i.tuted, it is called a _dry cell_.
Energy in a Cell.--A battery is made up of a number of these cells. Each cell has a certain electric energy, usually from one and a half to one and three-quarter volts, and from twenty-five to forty amperes.
The amperage of a cell depends on its size, or rather by the area of the electrodes; but the voltage is a constant one, and is not increased by the change, formation, or size of the electrodes.
For this reason the cells are used in groups, forming, as stated, a battery, and to get efficient results, various methods of connecting them up are employed.
[Ill.u.s.tration: _Fig. 35. Series Connection._]
Wiring Methods.--As at least six cells are required to operate a coil, the following diagrams will show that number to ill.u.s.trate the different types of connections.
Series Connection.--The six cells, Fig. 35, show the carbon electrodes A, of one cell, connected by means of a wire B with the zinc electrode wing C of the next cell, and so on, the cell at one end having a terminal wire D connected with the zinc, and the cell at the other end a wire E connected with the carbon electrode.
The current, therefore, flows directly through the six cells, and the pressure between the terminal wires D, E, is equal to the combined pressure of the six cells, namely, 1-1/2 6, which is equal to 9 volts.
The amperage, however, is that of one cell, which, in these diagrams, will be a.s.sumed to be 25.
[Ill.u.s.tration: _Fig. 36. Multiple, or Parallel Connection._]
Parallel Connection.--Now examine Fig. 36. In this case the carbon electrodes A are all connected up in series, that is, one following the other in a direct line, by wires B, and the zinc electrodes C, are, in like manner, connected up in series with each other by wires D. The difference in potential at these terminals B, D, is the same as that of a single cell, namely, one and a half volt.
The amperage, on the other hand, is that of the six cells combined, or 150. This method of connecting the cells is also called _parallel_, since the two wires forming the connections are parallel with each other, and remembering this it may be better to so term it.
Multiple Connections.--This is also designated as _series multiple_ since the two sets of cells each have the connections made like the series method, Fig. 35. The particular difference being, that the zinc terminals of the two sets of cells are connected up with one terminal wire A, and the carbon terminals of the two sets are joined to a terminal B.
[Ill.u.s.tration: _Fig. 37. Series-Multiple Connection._]
The result of this form of connection is to increase the voltage equal to that of one cell multiplied by the number of cells in one set, and the amperage is determined by that of one cell multiplied by the two sets.
Each set of cells in this arrangement is called a battery, and we will designate them as No. 1, and No. 2. Each battery, therefore, being connected in series, has a voltage equal to 4-1/2 volts, and the amperage 50, since there are two batteries.
Now the different arrangement of volts and amperes does not mean that the current strength is changed in the batteries or in the cells. If the pressure is increased the flow is lessened. If the current flow, or the quant.i.ty sent over the wires is increased, the voltage is comparatively less.
Watts.--This brings in another element that should be understood. If the current is multiplied by the amperes a factor is obtained, called _Watts_. Thus, as each cell has 1-1/2 volts and 25 amperes, their product is 37-1/2 watts.
To show that the same energy is present in each form of connection let us compare the watts derived from each:
Series connection: 9 volts 25 amperes, equal 225 watts.
Parallel connection: 1-1/2 volts 150 amperes, equal 225 watts.
Series Multiple connection: 4-1/2 volts 50 amperes, equal 225 watts.
From the foregoing, it will be seen that the changes in the wiring did not affect the output, but it enables the user of the current to effect such changes that he may, for instance, in case a battery should be weak, or have but little voltage, so change connections as to temporarily increase it, although in doing so it is at the expense of the amperage, which is correspondingly decreased.
It would be well to study the foregoing comparative a.n.a.lysis of the three forms of connections, so far as the energy is concerned, because there is an impression that increasing the voltage, is adding to the power of a current. It does nothing but increase the pressure. There is not one particle of increase in the energy by so doing.
[Ill.u.s.tration: _Fig. 38. Circuit Testing._]