3. A SWITCH COMPRISING MEANS FORMING A CLOSED CONTAINER, A PISTON WITHIN SAID CONTAINER TO DEFINE A FLUID RESERVOIR ADAPTED TO HOLD MERCURY AND A TEMPORARY HOLDING CHAMBER, FLUID PASSAGE MEANS INTERCONNECTING SAID TEMPORARY HOLDING CHAMBER AND SAID RESERVOIR FOR TRANSFERRING MERCURY FROM SAID RESERVOIR TO SAID TEMPORARY HOLDING CHAMBER AND FOR DRAINING THE TRANSFERRED MERCURY BACK TO SAID RESERVOIR, SAID PISTON BEING OPERABLE TO BE MOVED INTO SAID MERCURY IN THE RESERVOIR TO DISPLACE A VOLUME THEREOF TO TRANSFER SAME TO SAID TEMPORARY HOLDING CHAMBER, MEANS FORMED IN SAID TEMPORARY HOLDING CHAMBER SERVING TO REDUCE THE LATERAL CONFINES OF THE DISPLACED MERCURY TO ELONGATE SAID VOLUME IN A DIRECTION AXIALLY OF SAID CONTAINER TO A GREATER DEGREE IN SAID TEMPORARY HOLDING CHAMBER THAN IN SAID RESERVOIR, MEANS FOR MOVING SAID PISTON INTO AND OUT OF THE MERCURY IN SAID RESERVOIR TO TRANSFER SAME BETWEEN SAID RESERVOIR, MEANS SAID TEMPORARY HOLDING CHAMBER, A FIRST ELECTRODE PENETRATING INTO SAID RESERVOIR AND ADAPTED TO BE CONNECTED TO A SOURCE OF POTENTIAL, A PLURALITY OF SECOND ELECTRODES CARRIED BY THE CONTAINER, AND MEANS RESPONSIVE TO MOVEMENT OF SAID PLUNGER DISPLACING A PORTION OF SAID MERCURY TO SELECTIVELY CONNECT SAID FIRST ELECTRODE VIA SAID DISPLACED MERCURY TO A SELECTED ONE OF SAID SECOND ELECTRODES, TO THE EXCLUSION OF THE OTHERS.