A henry (H) is a unit of inductance, which is the mathematical representation of electromagnetic induction. The unit is named after American physicist Joseph Henry.[1] When 1 ampere (A) of electric current is changing per second (s) within an electric circuit, 1 henry of inductance will produce a voltage equal to 1 volt (V) across the inductor.[2] The units of a henry are then [math]H=\frac{V \cdot s}{A}[/math].

Since a changing magnetic field near a conductor will induce an electric current, as explained by Lenz's law, the changing current necessary for induction naturally comes as result. This can be explored using the PhET simulation below which was graciously provided to us by the University of Colorado. As the magnet's velocity varies near the inductor (the loop of wire), a voltage is induced across it which drives the current, and lights up the light bulb.

Visit Dr. Rowlett's unit dictionary for more information on the henry.


  1. Rowlett Unit Dictionary. (Accessed Sept 4, 2015). henry (H) [Online], Available: https://www.unc.edu/~rowlett/units/dictH.html#henry
  2. What Is. (Accessed Sept 4, 2015). henry [Online], Available: http://whatis.techtarget.com/definition/henry-H