beginning and end of winding one. He found that when the battery contact was broken, a shock could be felt between the first terminal of the first coil and the second terminal of the second coil. Further experimentation showed how the coil device could bring the shock from a small battery up the strength level of a big battery. So, Callan tried making a bigger coil. With a battery of only 14 seven-inch (178 mm) plates, the device produced power enough for an electric shock "so strong that a person who took it felt the effects of it for several days." Callan thought of his creation as a kind of electromagnet; but what he actually made was a primitive induction transformer. Callan's induction coil also used an interrupter that consisted of a rocking wire that repeatedly dipped into a small cup of mercury (similar to the interrupters used by Charles Page). Because of the action of the interrupter, which could make and break the current going into the coil, he called his device the "repeater." Actually, this device was the world's first transformer. Callan had induced a high voltage in the second wire, starting with a low voltage in the adjacent first wire. And the faster he interrupted the current, the bigger the spark. In 1837 he produced his giant induction machine: using a mechanism from a clock to interrupt the current 20 times a second, it generated sparks, an estimated 60,000 volts and the largest artificial bolt of electricity then seen. The 'Maynooth Battery' and other inventions Callan experimented with designing batteries after he found the models available to him at the time to be insufficient for research in electromagnetism. The Year-book of Facts in Science and Art, published in 1849, has an article titled "The Maynooth Battery" which begins "We noticed this new and cheap Voltaic Battery in the Year-book of Facts, 1848, p. 14,5. The inventor, the Rev. D. Callan, Professor of Natural Philosophy in Maynooth College, has communicated to the Philosophical Magazine, No. 219, some additional experiments, comparing the power of a cast-iron (or Maynooth) battery with that of a Grove's of equal size." Some previous batteries had used rare metals such as platinum or unresponsive materials like carbon and zinc. Callan found that he could use inexpensive cast-iron instead of platinum or carbon. For his Maynooth battery he used iron casting for the outer casing