Joseph Henry (1797-1878) discovered the phenomenon of mutual inductance independently and more or less simultaneously with Michael Faraday. Because Faraday published his results a little earlier, he received credit for the discovery. But due to the large volume and quality of his published research, Henry gained stature early in life. He was six years younger than Faraday, and outlived him by eleven years. Both were great experimenters, kept careful records and wrote for a broad audience.

Henry was a persistent builder of electromagnets. He built larger models with better windings, and he conceived of new applications such as the doorbell and magnetic relay, which Samuel Morse adapted to create his revolutionary telegraph.

Henry outlined the phenomenon of self-inductance. (The actual term “inductance” was first used a little later, in 1886, by Oliver Heaviside.) Any conductor through which current flows exhibits self-inductance and, if there is a second conductor nearby, mutual inductance. The phenomenon becomes greater if the conductor is formed as a coil and also if the coil surrounds a material, such as soft iron, that is permeable to a magnetic flux. In honor of Joseph Henry, the unit of inductance is known as the henry. The fundamental relationship between electricity and magnetism is given in the equation:

V(t) = L di/dt

Where V(t) = electrical force, V/sec; i = current, A; L = inductance, H. In words, a voltage is induced that is equal to the product of the inductance and current’s rate of change through the inductor. Notice that what is important about the electrical parameters is their rate of change.

A fluctuating current (in response to a changing applied voltage) continually constructs and deconstructs the magnetic field that surrounds the conductor. As the magnetic field builds, electrical energy is removed from the circuit, and this is manifest as inductive reactance, contributing to a rise in impedance. As the magnetic field collapses, a related amount of electrical energy returns to the circuit. The law of conservation of energy requires that this happen.

Using two analog inputs of an oscilloscope, one configured to see current, the voltage and current waveforms can be observed to be out of phase. It is said that in an inductor, the phase of the current lags the phase of the voltage. This phenomenon can be harmful or beneficial depending upon the design intent. It is always a consideration when trying to understand a circuit in which there is inductance and where the applied voltage fluctuates at a significant frequency, including dc energy that is abruptly switched on or off.

Joseph Henry’s research was of a practical nature. It remained in the years that followed for James Clerk Maxwell (1831-1879) and Oliver Heaviside (1850-1925) to develop more complete perspectives that are the basis for our present understanding of the intimate relationship between electricity and magnetism.