How did Hermann von Helmholtz measure the speed of nerve impulses?

Hermann von Helmholtz measured the speed of nerve impulses in 1850 by inventing the myograph, an instrument that allowed him to experimentally record the timing of these signals.

Prior to this, many scientists, including his teacher Johannes Müller, believed that nerve impulses occurred too rapidly to be measured and were governed by a mysterious "vital force." By demonstrating that the impulse was measurable and traveled at a relatively slow speed of approximately 90 feet (27 meters) per second, Helmholtz provided empirical evidence that nervous conduction could be explained by ordinary physical and chemical laws rather than vitalistic forces. This discovery was a significant step in refuting vitalism and supported the view that biological processes could be understood through the principles of classical mechanics.1