High Frequency Spark Tester consists of a Solenoid and an Interrupter which has natural High Frequency and when the current passes through the Solenoid, the hammer is attracted by its core, thereby breaking the circuit and at the same time charging the condenser.  The hammer then springs back reforming the contact and allowing the condenser to discharge through an inductance of a few turns of thick wire of the Tesla Coil.  This metal rod when brought within 1cm of the lining of the tank or any other vessel, produces a diffused Spray discharge, but if there is a minute crack, or microscopic hole in the lining, whether it be Glass or Rubber or PVC or any other insulation, this diffused discharge is concentrated into a bright spark passing through the hole or crack.  This phenomenon is attributed to the fact that High Frequency current possess a very high tendency of taking the nearest passage to Earth.  This method is therefore very useful in detecting faulty lining of vessels or tanks etc, or faulty insulation of cables, or for the detection of leaks and cracks in High Vacuum Systems, Scientific and laboratory glass apparatus