in the electric insulation layer and its insulation property was gradually degraded with the increase in immersion time in water as shown by curve a of fig4 in contrast a transformer coil electrically insulated by the method of this invention was provided with an excellent electric insulation layer which indicated no change in insulation resistance even when dipped in water for 150 hours as seen from curve b of fig4 a prescribed number of heating cycles were applied to the transformer coil wound with the electric insulation tape at room temperature and 220 u00b0 c a prescribed amount of current was let to pass through the transformer coil each time a prescribed number of heating cycles were carried out thereby measuring a temperature rise in said coil the result is shown in fig5 with the conventional electric insulation process represented by the foregoing control conduction of current led to a prominent temperature rise in the transformer coil as heating cycles increased in number as seen from curve c of fig5 in contrast the electric insulation process of this invention represented by example 2 did not cause a passage of current through the transformer coil to give rise to a temperature rise therein even when heating cycles increased in number as seen from curve d of fig5 proving that the electric insulation process of the invention exerted a full effect as mentioned above the electric insulation.