Measurements were made of the degree to which a lightning arrester could limit the voltage on avionic equipment when an external lightning arrester was struck by a simulated lightning stroke. The tests show that breakdown is not an instantaneous affair, but rather takes many microseconds. Measurements taken near a point which is struck indicate that the air around any protrusions will be in a state of electrical breakdown whenever the electrical field strength at the aircraft surface approaches 100 kVmeter. Electrical discharges tend to limit the field strength to that value, thus defining the electrical environment to which the avionics equipment is subjected. Data is presented showing how the impedance affects the voltages impressed on avionic equipment before the spark gaps in the protecting lightning arrester break down. Measurements were made of the spectral density of radiation from long electrical arcs used to simulate lightning strokes to aircraft. The relative amplitude at different frequencies seems to agree with that observed from natural lightning, falling at a 1f rate in the vicinity of 1 MHz.