That isn't the problem and the circuit can not possible work as you have it.
There are two major flaws in the design:
1. The output of the 555 has to be synchronous to the AC but it free runs so the triac trigger could be anywhere in the AC cycle.
2. The gate voltage has no reference, the current from the transistor to the gate has no path to follow.
To make it work so the variable resistor controls the current through the electromagnet, move it to the top side of the triac and connect the bottom of the triac to pin 1 of the 555.
Remove the connection from the bottom of the transformer to the load completely. That will let the 555 trigger the triac properly, at least in SCR mode.
The next problem is the 555 itself, it needs to work in monostable mode, not astable and the trigger signal to reset the monostable has to come from the transformer. The idea is that you create an adjustable delay from some part of the AC waveform, preferably the zero crossing point, then after that delay you trigger the triac.
To be honest, I think you would do better to remove the 555 completely and wire the circuit as a standard phase controller, it should work if you copy the circuit for a lamp dimmer.
Brian.