They sell DXSBV Double-Yoke Single-Tuning Adjustable Air Gap Electromagnets.

For 100 mm Petri dish can be used DXSBV-100.
Maximum magnetic field strength is 1.5 tesla.
Weight = 200 kg. Power = 2 kW.

But for 35 mm Petri dish can be used DXSBV-50.
Maximum magnetic field strength is 1 tesla.
Weight = 40 kg. Power = 300 W.

 

Neither of the devices shown in post#21 looks like the external magnetic field will extend very far. So any magnetic hazard would be rather in a very small area.