This exhibit is a 1915 Experimental Pliotron from the William White G E Museum Collection, Schenectady, New York. Tyne collection. It was never fitted with a base cap.
The lower tube is 25 mm in diameter.
Thanks to Joe Gruber for sending the image.
The balloon envelope is 50 mm in diameter and is 114 mm tall.
References: J Gruber & 1047. Type Pliotron was first introduced in 1915.