FYI - You should avoid firing an ignition (whether lawnmower or automotive) without a path to ground either through the specified spark plug gap or direct short to ground.
Here's why: The ignition coil can generate much higher voltage than it takes to fire the properly gapped spark plug. When the spark plug fires, the resistance of the ionized air in the spark plug gap is so low that the coil output voltage can't rise above the voltage that it took to ionize the air in the gap to initiate the spark. When the coil output wire is held out in the air, far from a ground, the voltage rises tens of thousands of volts higher than the spark plug gap allows. The insulation inside the coil cannot withstand that high of a voltage and will arc through the insulation of the coil - which blows a pin hole through the insulation, leaving carbonized (conductive) material coating the inside of that hole - so the coil is now defective - no longer allows the coil output voltage to rise to the ionization voltage of the proper spark plug gap.