21. This electromagnetic force , E, will in turn drive a current of density j in closed loops. 22. The photon, the particle of light which mediates the electromagnetic force is believed to be massless. 23. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force . 24. It has been established that the powder was more sensitive to gravitational forces and electromagnetic forces . 25. A few years ago, I tried to explain gravitational force as caused by the electromagnetic force . 26. For high-speed precision scales, a load cell using electromagnetic force restoration ( EMFR ) is appropriate. 27. Electromagnetic forces are tremendously stronger than gravity but cancel out so that for large bodies gravity dominates.28. But its versions of the strong, weak and electromagnetic forces would be different from those we know. 29. The Kaluza-Klein theory used the fifth dimension to unify gravity with the electromagnetic force ; e . g. 30. Molecular and quantum physics show that the electromagnetic force is the fundamental interaction responsible for contact forces.