English: Early induction coil, 1838, by Charles G. Page (US). Exhibit in National Museum of American History, Washington, DC, USA. The arm dipping into the glass cup was an early "contact breaker" or "interrupter" which repeatedly broke the current to the primary to create the flux changes needed for induction. The cup was filled with mercury, and the end of the wire from the battery was in it, acting as a switch to complete the primary circuit. When battery power was supplied current flowed through the arm to the coil primary, creating a magnetic field in the coil. This attracted the iron piece on the left end of the arm next to the coil. This lifted the right end of the arm out of the mercury cup, breaking the primary circuit. The magnetic field in the coil collapsed, releasing the arm, which fell back into the mercury, completing the circuit again, and the cycle repeated. Each time the arm lifted out of the mercury, breaking the primary circuit, the magnetic flux change induced a high voltage pulse in the secondary of the coil.