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Glossary of terms you will come across during your Phase 2 training.

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In the SI, there are 7 base units of measurement. The ones we come across in the phase 2 course are:

Other units of measurement such as those for torque, speed, power etc. are all based on SI units of measurement, so they are said to be 'base derived' units of measurement.


A base limiting resistor is inserted into an electronic circuit in order to protect the delicate base/emitter circuit of a transistor. Unlike the more robust collector/emitter, the base/emitter circuit cannot pass large currents, it can only pass small currents. The transistor works by taking the small base/emitter current and amplifying it-allowing a much larger collector emitter current to flow.

In order to limit the current flowing into the base of a transistor, we fit a resistor in series into the circuit. This has the effect of limiting the current flowing into the base/emitter circuit of the transistor.

A base limiting transistor.

Transistors need not always have a base limiting resistor fitted. Sometimes the incoming current (or signal) is so small that there is no danger to the transistor. In these cases, the function of the transistor is usually to amplify the signal so that it can be used by other parts of the electronic system.


In DC electricity, a single source of that energy is known as a cell.

When we join cells together in series, parallel or series parallel, we then have a battery of cells, or just simply a battery.

When deciding to join single cells in series or in parallel, we consider what we are trying to achieve:

  • If we want to increase the available capacity of a cell, we add in more positive and negative plates in parallel with each other into the cell.
    Two 12 volt batteries connected together in parallel to give an overall 12 volts, but also adding the capacity of one battery to the otherOne example of this arrangement would be where we are trying to start a vehicle with a flat battery by 'jump starting' the vehicle. What we do here is that we get a vehicle with a charged battery and connect the two vehicle batteries together in parallel (positive to positive, negative to negative). By doing this, we don't change the overall voltage of either vehicle, we are simply adding the capacity of the 'good' battery to that of the 'flat' battery, and using the additional capacity to start the second engine.
    Another example of this arrangement
    is where we want to increase the capacity of a 12V lead/acid battery. Instead of having just one positive and one negative plate, we commonly put 10 or more negative and positive plates in parallel with each other into the cell. This has no effect on the cell voltage, but does significantly increase cell capacity.
  • If we want to increase the available voltage, we join cells in series. The overall capacity of each cell joined in series remains the same.
    Six individual two volt cells arranged in series with each other to give an overall 12 voltsAn example of this series arrangement would be in a 6-cell lead/acid battery. Each individual cell has a nominal voltage of 2V, and when they are joined together in series, these 6 cells in the battery give an overall nominal voltage of 12V.
    Another example of this series arrangement would be connecting four 6 volt batteries together in series, giving us an overall voltage of 24 volts.

HydrometerA hydrometer is an instrument for measuring the density of a liquid relative to that of water.

During your phase 2 course, you will use hydrometers on two occassions:

  1. Measuring the relative density of battery electrolyte, in order to check the state of charge of the battery.
  2. Measuring the relative density of engine coolant, in order to check the concentration of anti-freeze in the coolant.

On the right we can see an actual hydrometer. It is a floating device. The denser the liquid, the higher it will float, and give a reading of the liquid density on its graduated stem.

The instrument is basically a glass tube with lead shot at one end and a graduated scale at the other.

The instrument is used to compare the density of whatever liquid it is immersed in to the density of water. Water is said to have a density of one, so the electrolyte from a fully charged battery would be 28% more dense-1.28.

Functions, Operation and Use of the Hydrometer:

The hydrometer is used in plant vehicle electrics to measure the density of battery electrolyte compared to that of water. Battery electrolyte in a fully charged battery is about 25% more dense than water.

We can use the hydrometer to measure the density of the battery electrolyte compared to that of water. This reading is dependent on the ambient temperature, and the following figures are for a temperature of 250C:

Hydrometer completely assembled

Exploded view of hydrometer

Hydrometer scale

Using the hydrometer:

Personal Protective Equipment (PPE) to use when working with batteries.Always use safety glasses or a full face mask when working with batteries and electrolyte. Make use also of the acid proof gloves and aprons provided. If you get battery acid on your skin or in your eyes, flush immediately with copious amounts of water. If pain and discomfort persist, seek medical assistance. Check here for emergency procedures.

Go to each cell in turn. Agitate the electrolyte in the cell. Then take the electrolyte into the instrument. If any of the battery plate active material has broken away, the electrolyte will be discoloured, so make sure the electrolyte is clear.

Induce enough electrolyte into the instrument to ensure the hydrometer is fully afloat. Be careful not to induce too much electrolyte, as it would cause the hydrometer to rise too much and strike the top of the instrument, giving you a false reading. Make sure the hydrometer is fully and freely afloat.

Read off on the scale provided to see the relative density of the elctrolyte.

During the induction phase of the course, you will be issued (at your own expense) with a battery hydrometer.


Emile BaudotIn telecommunications and electronics, baud means the same thing as symbols per second or pulses per second.

It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes (signaling events) made to the transmission medium per second in a digitally modulated signal or a line code.

The baud unit is named after Emile Baudot, the inventor of the Baudot code for telegraphy, and is represented as SI units are - the first letter of its symbol is uppercase (Bd), but when the unit is spelled out, it should be written in lowercase (baud) except when it begins a sentence.

Read more about baud on Wikipedia


Engine top dead centre, bottom dead centreBottom Dead Centre: The instant at which the piston in an engine has reached its lowest point within the cylinder. It is called "dead" centre, because the piston has finished its downwards movement, and stops (hence the use of the term "dead") before commencing its upward movement.

At BDC, the small end, big end and main bearings are in one straight line.


Straight cut bevel gear, used to turn drive through 90 degrees.

Bevel gears are used to turn drive through 900.

Brake Horse Power: The amount of power that is available at the flywheel of an engine. Power is generated in the combustion chambers, but due to mechanical inefficiencies and losses, not all the power becomes available at the flywheel. The amount of power at the flywheel is measured by running the engine against some form of absorbtion brake, hence the term BRAKE Horsepower.
In the context of the study of DC electricity and electronics, bias describes when we deliberately apply a certain DC voltage to one electrode relative to another.
For example, if we apply a certain voltage to the anode of a diode that is higher than the voltage on the cathode, we have 'forward biased' the diode.
Once this voltage bias exceeds a certain value (the forward voltage of the diode), current will flow through the diode.

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