evaporated the method of the present invention utilizes the characteristic impedance alternation of the cable due to the impregnation of the liquid to be detected into the porous insulation the alteration is measured by transmitting a pulse wave from one end into the cable then analyzing the deformed wave reflected from the point where the characteristic impedance has been altered by the impregnation of liquid i e measurement of voltage standing wave ratio vswr the method of the present invention will be further explained using fig4 a and b in fig4 a the leak detection cable 31 consists of two parallel conductors 32 and 33 one end of which being connected to a pulse generator 34 and a receiver oscilloscope 35 connected in parallel to the generator and the other end being terminated by a resistor 36 upon sending a pulse wave with a period t sec and pulse width u03c4 sec into the cable the pulse propagates at a speed peculiar to the wave generally 50 80 of the light speed a reflected pulse wave is shown on a receiver screen 35 as a trapezoid while there is no impregnation of liquid into an insulation 33 of the cable 31 if an impregnation occurs at a point 37 in the cable 31 the characteristic impedance of the cable is changed and some pulse is reflected back to the receiver 35 the reflected pulse wave when amplified and projected on a braun tube as a function of time synchronized with the input pulse period t exhibits a pulse wave as shown in fig4 b the distance between the end 38 and the impregnated point 37 is calculated from the following equation.