The German physicist Heinrich Hertz (1857–1894) was the first to generate and detect certain types of electromagnetic waves in the laboratory. Starting in 1887, he performed a series of experiments that confirmed the existence of electromagnetic waves and verified that they travel at the speed of light. Hertz used an alternating-current RLC (resistor-inductor-capacitor) circuit that resonated at a known frequency and connected it to a loop of wire. High voltages induced across the gap in the loop produced sparks that were visible evidence of the current in the circuit and helped generate electromagnetic waves. Across the laboratory, Hertz placed another loop attached to another RLC circuit, which could be tuned (like a dial on a radio) to the same resonant frequency as the first and could thus be made to receive electromagnetic waves. The spark was generated in the receiver loop, indicating that electromagnetic waves travel toward the receiver loop. Hertz also studied the reflection, refraction, and interference patterns of electromagnetic waves, confirming their wave characteristics. Hertz was able to determine the wavelengths from the interference patterns, and by knowing their frequencies, the propagation speed was calculated using the equation v = fλ. Hertz was thus able to prove that electromagnetic waves travel at the speed of light. The SI unit for frequency, the Hertz (1 Hz = 1 cycle/second), is named in his honor.