迈克尔·法拉第所引申的概念力线（英语：Line of force），也被詹姆斯·克拉克·麦克斯韦称为感应线（英语：Line of induction）。根据约瑟夫·汤姆森，法拉第通常把力线定义为介电质中被极化粒子间的“联系”，不过他有时将它作为这些粒子穿过真空延伸的方向。像麦克斯韦一样，汤姆森还还把电磁感应中的“管”（tubes）叫做力线。从20世纪的观点来看，力线是19世纪统一场论中的能量内在连接，而且这引发了数学上、实验上都更为成熟的概念和理论，包括麦克斯韦方程组、电磁波和阿尔伯特·爱因斯坦的相对论。

A line of force in Faraday's extended sense is synonymous with Maxwell's line of induction. According to J.J. Thomson, Faraday usually discusses lines of force as chains of polarized particles in a dielectric, yet sometimes Faraday discusses them as having an existence all their own as in stretching across a vacuum. In addition to lines of force, J.J. Thomson—similar to Maxwell—also calls them tubes of electrostatic inductance, or simply Faraday tubes. From the 20th century perspective, lines of force are energy linkages embedded in a 19th-century unified field theory that led to more mathematically and experimentally sophisticated concepts and theories, including Maxwell's equations, electromagnetic waves, and Einstein's relativity.