Spark-gap transmitter

A spark-gap transmitter consists of a spark gap connected across an oscillatory circuit consisting of a capacitor and an inductor in series or parallel.

In a typical transmitter circuit, a high voltage source(e.g. a battery, or a high voltage transformer) charges a capacitor (C1 ) through a resistor until the spark gap discharges(due to ionization), then a pulse of current passes through the capacitor (C2). The inductor and capacitor after the gap form a resonant circuit. After being excited by the current pulse, the oscillation rapidly decays because energy is radiated from the antenna. Because of the rapid onset and decay of the oscillation, the RF pulse occupies a large band of frequencies because due to sudden flow of large current, free electrons of antenna become capable of leaving metallic surface and starts moving to some extent so as to generate EM waves.

The function of the spark gap is to present a high resistance to the circuit initially to allow the capacitor to charge. When the breakdown voltage of the gap is reached, it then presents a low resistance to the circuit causing the capacitor to discharge(due to short circuiting). The discharge through the conducting spark takes the form of a damped oscillation, at a frequency determined by the resonant frequency of the LC circuit

A simple spark gap consists of two conducting electrodes separated by a gap immersed within a gas. When a sufficiently high voltage is applied, a spark will bridge the gap, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current is reduced below a minimum value called the 'holding current'. This usually occurs when the voltage across the gap drops sufficiently, but the process may also be assisted by cooling the spark channel or by physically separating the electrodes. This breaks the conduction through  ionized gas, allowing the capacitor to recharge, and permitting the recharging/discharging cycle to repeat. The action of ionizing the gas is quite sudden and violent , and it creates a sharp sound . The spark gap also liberates light and heat.

Electrophorus

An electrophorus is a capacitive  generator used to produce electrostatic charge via the process of electrostatic induction.

The basic components of an electrophorus are a flat, plate-shaped insulator, an insulating handle and a metal disk. The insulator, usually called a “cake,” can be made from a variety of materials. Unlike the cake, the metal disk is a conductor. In the electrophorus, the disk acts as an electrode by allowing current to pass through to a nonmetallic medium. The metal disk of an electrophorus attaches to an insulating handle and the cake stands alone.

The dielectric plate is charged through the by rubbing it with fur or cloth. Let us suppose that  the dielectric gains negative charge by rubbing. Then, the metal plate is placed onto the dielectric plate. Due to electrostatic induction it develops two regions of charge — the positive charges in the plate are attracted to the side facing down toward the dielectric, charging it positively, while the negative charges are repelled to the side facing up, charging it negatively, with the plate remaining electrically neutral as a whole. Then, the side facing up is momentarily grounded (which can be done by touching it with a finger), draining off the negative charge. Finally, the metal plate, now carrying only positive sign of charge  is lifted.

Since the charge on the dielectric is not depleted in this process, the charge on the metal plate can be used for experiments, for example by touching it to metal conductors allowing the charge to drain away, and the uncharged metal plate can be placed back on the dielectric and the process repeated to get another charge. This can be repeated as often as desired, so in principle an unlimited amount of induced charge can be obtained from a single charge on the dielectric. In actual use the charge on the dielectric will eventually  leak off through the surface of the dielectric  or the atmosphere to recombine with opposite charges around to restore neutrality.

Versorium

The first electrical engineer was probably William Gilbert who designed the versorium : a device that detected the presence of statically charged objects. The versorium is a needle constructed out of metal which is allowed to pivot freely on a pedestal.  The versorium is of a similar construction to the magnetic compass, but is influenced by electrostatic rather than magnetic forces. The needle is attracted to charged bodies brought near it, turning towards the charged object.The needle turns to point at a nearby charged object due to charges induced in the ends of the needle by the external charge, through electrostatic induction.

The theory of electrostatic induction says  “A normal uncharged piece of matter has equal numbers of positive and negative electrical charges in each part of it, located close together, so no part of it has a net electric charge. When a charged object is brought near an uncharged, electrically  conducting object, such as a piece of metal, the force of the nearby charge causes a separation of these charges”. For example, if a positive charge is brought near the object  the negative charges in the metal will be attracted toward it and move to the side of the object facing it, while the positive charges are repelled and move to the side of the object away from it. This results in a region of negative charge on the object nearest to the external charge, and a region of positive charge on the part away from it. These are called induced charges. If the external charge is negative, the polarity of the charged regions will be reversed. Since this is just a redistribution of the charges that were already in the object, the object has no net charge. This induction effect is reversible; if the nearby charge is removed, the attraction between the positive and negative internal charges cause them to intermingle again. 

It should be kept in mind that  only the negative charges in conductive objects, the electrons, are free to move.

Commutator

A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor's armature, a steady rotating force (torque) is produced.  The current in the winding causes the fixed magnetic field to exert a rotational force (a torque) on the winding, making it turn. As the rotor's field comes close to aligning itself with that of the stator, the commutator switches the rotor's polarity, so the motor is perpetually trying to settle.

The armature is connected to the commutator which rides along the brushes which are connected to a DC power source. The current from the DC power source flows from the positive lead, through the brush labelled   A1 through one commutator section, through the armature coil, through the other commutator section, through the brush labelled  A2 and back to the negative lead. This current will generate lines of flux around the armature and affect the lines of flux in the air gap. On the side of the coil where the lines of flux oppose each other, the magnetic field will be made weaker. On the side of the coil where the lines of flux are not opposing each other, the magnetic field is made stronger. Because of the strong field on one side of the coil and the weak field on the other side, the coil will be pushed into the weaker field and, because the armature coil is free to rotate, it will rotate.

In practice, DC motors will always have more than two poles. In particular, this avoids "dead spots" in the commutator. In case of two pole rotor, when the rotor will exactly at the middle of its rotation (perfectly aligned with the field magnets), it will get "stuck" there. Meanwhile, with a two-pole motor, there is a moment where the commutator shorts out the power supply (i.e., both brushes touch both commutator contacts simultaneously). This would be bad for the power supply, waste energy, and damage motor components as well. Another disadvantage of such a motor is that it would exhibit a high amount of torque "ripple" (the amount of torque it could produce is cyclic with the position of the rotor). In this type of motor ,one pole is fully energized at a time (but two others are "partially" energized). As each brush transitions from one commutator contact to the next, one coil's field will rapidly collapse, as the next coil's field will rapidly charge up.

Traction motor

Traction motor consists of two parts, a rotating armature and fixed field windings surrounding the rotating armature mounted around a shaft. The fixed field windings consist of tightly wound coils of wire fitted inside the motor case. The armature is another set of coils wound round a central shaft and is connected to the field windings through "brushes" which spring-loaded contacts pressing against an extension of the armature are called the commutator. The commutator collects all the terminations of the armature coils and distributes them in a circular pattern to allow the correct sequence of current flow. It has a low resistance field and armature circuit. Because of this, when voltage is applied to it, the current is high. The advantage of high current is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting a train. The disadvantage is that the current flowing into the motor has to be limited, otherwise the supply could be overloaded and  its cabling could be damaged. At best, the torque would exceed the friction and the driving wheels would slip. Resistors are use to limit the initial current. As the DC motor starts to turn, the interaction of the magnetic fields inside causes it to generate a reverse voltage internally. This "back-EMF" (electromagnetic force) opposes the applied voltage and the current that flows is governed by the difference between the two. As the motor speeds up, the back-EMF rises, the resultant EMF falls, less current passes through the motor and the torque drops. The motor naturally stops accelerating when the forces due to weight of the train match the torque produced by the motors. To continue accelerating the train, series resistors are switched out step by step, each step increasing the effective voltage and thus the current and torque for a little bit longer until the motor catches up. When no resistors are left in the circuit, full line voltage is applied directly to the motor. If the train starts to climb an incline, the speed reduces because forces due to weight of the train is greater than torque and this reduction in speed causes the back-EMF to fall and thus the effective voltage to rise - until the current through the motor produces enough torque to match the new require force.