All English cars were equipped with a generator [Dynamo], starter motor, battery and lamp sand all were  wired to connect these together.

The function of the generator is to supply current to the electrical equipment.

The early cars were fitted with a third brush generator to control the output of the generator at various engine speeds, this third device was arranged to give alternative outputs, when running in daylight half can be used  with full charge for night driving. The third bush could be moved to give the required output when the lighting circuit was not in use . Between the generator and the battery a cut out served the purpose of preventing current from the battery leaking to earth via the generator windings.

Later on the generators were made with only two brushes and were controlled with a voltage control unit which also had a cutout fitted. It became apparent a current control device was also required, cars of the late fifties and early sixties were fitted with a combination control device controlling both voltage and current.

The starter motor consisted of a high current and a torque D.C. motor control by crude switches later on these crude switches were replaced by a solenoid which reduced the large current flow from going through the starter switch circuits.

The Battery circuit could consist of either a six or twelve volt battery and wiring connecting either the positive or negative terminal to earth connection can cause problems when fitting modern radios the earthing of the battery caused elusive problem.

The earthing cable was connected directly to the chassis. I would suggest that early cars should also have an earth cable to the engine block. All later English cars were fitted with the engine earth cable. As chassis are made of steel voltage drops can occur which does cause the odd lighting problems, these problems were most noticeable in cars using six volt batteries.
 
EXAMPLE:-Fully charged six volt battery No. of cells = 3
Voltage per cell say 2.3 volts = 5,9 volts.
Resistance of clean chassis = 0.1 ohm
tail light say using 3 amps.
Ohms law
 Voltage drop 3/.1 = .3 volt
So voltage at lamp = 6.9 - .1 = .3  = 6.6 volts.
Rusty chassis say resistance increases to 1 ohm
Voltage drop  = 3/ 1 = 3 volt.
Voltage at globe 6.9 - 3 = 3.9 volts hence dim tail lights.
Solution run separate earth wire or increase battery voltage to 8 volts as was done for Volkswagen.
Voltage per cell 2.3 / 4 = 9.2 Rusty chassis 1 ohm 9.2 -3 = 6.2 better lighting.

IMPROVING THESE ELECTRICAL SYSTEMS.

One major problem on early cars fitted with three brush generator and cut out was the cut out coil and contacts. The cut out coil [ reverse relay] can be replaced by a diode most diodes capable of 10-20 amps can be used.

Third brush generators caused few problems, time however caused the insulation on the field windings to break down causing some dramas the field windings are simple windings, careful removal and some new cotton via the wives dressmaking goodies wound around the field windings can do wonders.
The adjustment of third brush generators requires a certain amount of skill the tools needed for adjustment are a hydrometer and ammeter. Begin the adjustment by moving the third brush toward the output brush until a reading of approximately 4 amps is noted on the ammeter. Run the car for a few days and make a hydrometer reading it should be around 1. 250 this third brush regulation is current sensitive if the battery is down in condition you run the risk of increased voltage which overheats the field coils and could cause failed field windings.

The other cause of field over heating was a failed resistor in the lighting circuit this resistor was attached to the field coil at the opposite end of the third brush.

Most of these problems were overcome with the introduction of the contact voltage and current controlling device.

On some American cars the third brush generator was retained but the third brush was not adjustable. A two unit controlling device was fitted.

One problem with all these electrical systems is that people add extra loadings onto the systems i.e. heated rear window, brighter lights etc. in most cases the wiring generator switches etc. cannot cope , hence the need to fit alternators but thatís another story.
 

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