Atmospheric Waves and Currents Recorded by Electromagnetic Barographs

Two electromagnetic barographs designed by H. Benioff to respond to pressure variations in the frequency range from five cycles per second to one cycle in thirty seconds approximately and with sufficient sensitivity to record the natural unrest of the atmosphere have been in operation at the California Institute of Technology in Pasadena for approximately two years. These instruments were operated either at the same point with different characteristics or at points separated by distances from a few to more than 100 meters with identical characteristics. In the short period galvanometer combination the response is approximately proportional to the rate of change of pressure. The instruments respond to waves as well as to current variations. Discrimination between these two types of movements is made on the basis of their difference in velocity of propagation, the waves being propagated with the velocity of sound while current variations are propagated with much smaller velocity.