Modern Electrics, July, 1908, page 139:

WIRELESS  REGISTRY.

    This Department has been started with the idea to bring the wireless amateur in closer touch with commercial land and ship stations. Each month a list of new members will be printed here and once each year an official BLUE  BOOK will be issued by MODERN  ELECTRICS, giving a list of all the members who registered during the year. Each member will receive the Official Blue Book free of charge. The Blue Book will also contain a complete list of commercial and government stations, their call letters, wave length, etc.
    To register a station requires: Total length of aerial (from top to spark balls), spark length, call letter (if none is in existence M.  E. will appoint one), name and address of owner.
    Fee for Registry (including one Blue Book) 50 cents.
    For other particulars see June issue of this magazine.
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NAME  AND  ADDRESS  OF  OWNER.        CALL
LETTERS.
       APPROXIMATE
WAVE  LENGTH
IN  METERS.
       SPARK LENGTH
OF  INDUCTION
        COIL.        
  1--A. C. Austin, Jr., Hasbrouck Hgts, N. J. AU 200 2  ins.
  2--Otto E. Curtis, Rochester, N. Y. Q 152 1  "
  3--M. D. Douglas, Hasbrouck Hgts, N. J. MD 100 ¼  "
  4--Harry Gross, Hackensack, N. J. HG 125 ½  "
  5--Albert Higson, Jersey City, N. J. AH 75 1  "
  6--Harold E. Peck, Providence, R. I. PE 40 1  "
  7--J. Peters, Jr., Florissant, Mo. PI 125 4  "
  8--L. S. Stevens, Marlboro, Mass. LS 60 1½  "
  9--Newell A. Thompson, Brookline, Mass. KN 400-700 ½ KW
10--Earl Vogel, Ashton, Ill. AN 56 2  ins.
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RATING  SPARK  COILS.
    A good many amateurs labor under the wrong impression that the actual length of the spark of an induction coil is computed by the distance between the two spark balls.
    This, of course, is erroneous. The length of the spark must be measured between two sharp metal points, as for instance a 1 inch coil will hardly ever give more than spark, measured between the balls. This is on account of the balls having a large capacity, which cuts the spark length down.