Be carful when applying votages the secondary. You could end up with a very large voltage on the primary if it was a transformer intended for mains step-down. It's the turns ratio of the transformer that gives a good idea. If it is 20:1 there will be 20 times the voltage on the high turns side compared to the low turns side. A car HT coil (which is a transformer) produces 15 to 20 thousand volts form a 12 volt car battery. Remember we are talking about AC currents normally with relation to transformers. Be careful out there... your first electric shock could be last thing you feel !!