a device used to disable hypodermic needles uses a power supply and two electrodes in a housing . one electrode is placed over the other electrode and slants down towards the other . a collar is used to ensure that when the hypodermic needle is inserted in the device , it will hit the electrodes at the proper point . when the hypodermic needle contacts both electrodes , electricity will flow from the power supply through the lower electrode and up the needle to the top electrode . the electrical resistance of the needle is very high so that it quickly heats up to cause disintegration of the needle .