tesla coil(n = noun.artifact) - a step-up transformer with an air core; used to produce high voltages at high frequencies;
is a kind of transformer

   A transformer without iron, for high frequency alternating or oscillating currents; an oscillation transformer. [Webster 1913 Suppl.]

choke coil, choking coil, coil, coil spring, field coil, ignition coil, inductance coil, induction coil, nikola tesla, primary coil, reactance coil, rotor coil, ruhmkorff's coil, secondary coil, spark coil, stator coil, tesla, tesla transformer, tickler coil