Have a look at the small wire to the alternator. This is the excitation feed to the alternator. It is fed with 12 volts through the ignition warning light when the ignition is on. This voltage creates a magnetic field through the field windings, allowing the alternator to charge at low rpm. Above a certain rpm, the alternator will self excite and does not need this voltage. This then feeds 12 volt onto the excitation wire and thus the ignition warning light goes out. A dodgy connection will still allow the bulb to light but may not give enough power to excite the field windings.