The AC surface spark-over voltages on solid insulator with back electrode i
n liquid N-2 (LN2) and liquid He (LHe) were measured and discussed. The lar
ger gap length did not give a higher spark-over voltage. The spark-over vol
tage in LHe was about two-thirds of that in LN2. Experimental studies were
carried out on how bubbles in liquid influence surface spark-over voltage.
The result shows that it does not influence the voltage.