when is about frequency, things start to be little bit more complicated. you have 50 Hz (or 60 if you are in America or Japan) from your supply. or you have this thing:
Induction Coil, Box Form: 10mm spark gap
arc generator is working on high voltage so therefore you need transformer ("coils"). which is working only on AC so if you have DC input you need either inverter of some kind or in case of that device I put link above, there is high frequency switch which is cutting the supply to first coils so induction in secondary is possible. in case you can adjust speed of that switch (usually not), you can control frequency. otherwise, you need VFD (variable frequency drive) which is not so cheap -
Variable-frequency drive - Wikipedia, the free encyclopedia but with that you can control both frequency and voltage (depends on the model). controlling frequency is not simple and cheap as controlling the voltage
why do you want to control volts anyway? if you have too low voltage, thing will not work and if you have too high, you can burn secondary on transformer. you can control voltage on several ways. with controlling input voltage. if you have DC supply, you can buy some of those power supplies with changeable output from 1,5 to 12 V so you have controllable input (which will also result with output what you can calculate if you know coil data - number of coil turns for both coils - U2=U1N1/N2). otherwise you can control input voltage with some other more expensive DC sources with knob.
if you have AC input from home socket, you can control voltage with some resistors and make home made (just note that even 200V or 110V can, if not kill you, make you feel very nice. believe me, I had pleasure of enjoying that plenty times, from 50 to 690 V) controller
current depends on resistance and voltage. so if you put too high voltage, you can burn the coil. setting correct distance between anodes is also important. if they are too close, current will be high so you can also fuck up the coil. note that this is all happening on secondary so if you screw something, breaker or fuse on house wiring will not protect you!
HD transformers can be found in old CRT screens and TVs. there is 20 000 V and touching end of tube is not pleasant. as well as those arc generators who have 3000 V minimum
if you play with capacitors to make arc, remember that's more dangerous than transformers
hope some of this info will help... feel free to ask me. and as qualified electrical engineer, I should warn you not to play with your life
good luck
