32 HEYL & BRIGGS— THE EARTH INDUCTOR COMPASS. 



is never finished. Metallic brushes will probably be preferable to 

 carbon for a long flight ; and a commutator of hardened steel will 

 probably save frequent replacements. The question of brush wear 

 is naturally more important at eight (8) millivolts than at ordinary 

 commercial pressures. 



Addenda. 



Since the date of the presentation of the foregoing memoir certain 

 additional and correctional material has accumulated. 



A comparative examination of various forms of commutator and 

 brushes has shown that a mica-filled brass commutator and brushes 

 of carbon are by far the most satisfactory combination. Such an 

 arrangement has been run in the laboratory at 20 revolutions per 

 second for 146 consecutive hours with a variation of less than one 

 degree in the compass reading at any time during the test. The wear 

 on the brushes was trifling. One hundred hours' flying is about the 

 maximum service obtained from an airplane before complete over- 

 hauling of its engine is necessary. These additions have been incor- 

 porated in the text. 



U. S. Bureau of Standards, 

 Washington, D. C, 

 November, 1921. 



'■ This instrument is a part of the programme set by the Air Service, in 

 its attempt to put the navigation of the air on a basis as trustworthy as that 

 of the water. The study of the problem was undertaken by the Bureau of 

 Standards at the request of the Air Service, and the expense incident to the 

 development of the final successful model was defrayed by Air Service funds. 

 The flying tests were carried out with the cooperation of the Engineering 

 Division of the Air Service at McCook Field, Dayton, Ohio." 



