The voltage on conventional coils is provided by the Battery and a Ballast Resistor. The presense of the ballast resistor (or ballast resistor wire) is why you don't want to connect anything except a tach designed for this terminal to it. Here's what happens if you do. The voltage across the ballast resistor is E=IR where E is the voltage across the ballast resistor, I is the coil current, and R is the ohms value of the ballast resistor. If for example the coil current is 3 amperes and the resistor is 1 ohm, E is 3 vlots. If the battery voltage is 12 volts, the remaining coil voltage is 9 volts (12 - 3 volts = 9 volts). Now if you add the electric choke at the coil terminal, the current in the choke adds to the current in the coil, hence if the choke requires 2 amps to operate then the the combined current is 3+2 amps, i.e. 5 amps. 5 amps x 1 ohm = 5 volts, now the coil voltage is 12-5 volts = 7 volts instead of 9 volts. You've reduced the coil voltage by 2/12 or 18.33%. You've also reduced the spark output by 18.33%. The bottom line here is you don't connect anything to the coil BATT terminal on a conventional ignition system.