Approaching the circuit area the pilot selected the landing gear down, but the appropriate gear position light did not illuminate. The pilot then noticed that all electrical systems were inoperative. He subsequently advised that he checked the mechanically operated position indicator, and was satisfied that the gear was down. Witnesses observed the aircraft making a normal approach but then saw the gear collapse shortly after touchdown. Initial investigation revealed that the aircraft battery was fully discharged. The aircraft alternator had failed some time previously, and the battery had been steadily depleted. However, this situation would not have been evident to the pilot, as the ammeter was defective and showed a steady charge at all times. The last part of the gear extension cycle results in a very small movement of the position indicator, and it is considered difficult to assess the precise position of the gear by reference to the indicator. The aircraft handbook warns that a discharged battery may prevent the gear from fully extending by electrical power. The pilot was aware of this warning, but had not employed the emergency lowering procedure to ensure that the gear was locked down.