The aircraft was on a maintenance flight to check a recently installed engine oil temperature gauge. After determining that the gauge was operating satisfactorily, the pilot began a descent toward the airport. While descending through 3000 ft, the pilot observed white smoke in the cabin and smelled fumes from a possible electrical fire. He turned the electrical master switch off, lowered the gear manually and continued his descent to the airport. After landing, the aircraft was quickly taxied off the runway and the fuel selector was turned off. Shortly thereafter, fire developed on the left side in the area of the firewall and engine compartment. The fire spread rapidly and destroyed the aircraft. Reportedly, the aircraft had been having electrical problems in the past. The engine had been hard to start and a heavy load had been observed on the ammeter during landing gear cycles. Due to the extensive fire damage, the cause of the electrical fire could not be verified.