The artifact is a spark-gap voltmeter.
Built by Westinghouse in 1925, this 24-foot-tall device was used to test voltages at TVAs power equipment service shops in Muscle Shoals, Alabama. When a designated voltage was reached, a spark jumped between the two brass spheres, generating a burst of homemade thunder and lightning. The voltmeter could measure up to 500,000 volts of electricity. This machine, currently housed in the foyer of TVA offices in Chattanooga, is one of only a few spark-gap voltmeters still in existence in this country.