102 
MARVIN. 
and the transformer of the electrician. Finally the discovery 
of the X-ray phenomena and the recent progress in wireless 
telegraphy have each created a new demand for the induc¬ 
tion coil in its original form, and manufacturers have been 
stimulated to renewed activities in perfecting its construction. 
Prior to the introduction of dynamo-electric machinery the 
Spottiswood coil appears to have been by far the most power¬ 
ful one made up to that time. This coil was 4 feet long and 
20 inches in diameter. The secondary contained 280 miles 
of fine copper wire wound in four sections. With 30 cells of 
Grove battery the coil gave a spark 42 inches long. During 
the past year Queen & Co., of Philadelphia, have designed 
and built two very large coils for the Japanese government 
for use in wireless telegraphy.* It is claimed the new coils 
contain only 100 pounds of wire in the secondary circuit, 
as compared with 280 pounds in the old coil, and neverthe¬ 
less give 45-inch sparks with a current of 20 amperes and 
25 volts in the primary. The exact details of design of the 
new coils have not been made public, but the following dis¬ 
tinctive features are to be noted: The soft-iron core is rela¬ 
tively very large, 5 inches in diameter and 4 feet long, and 
weighs over 200 pounds. This is about five times as heavy 
as the core of the Spottiswood coil. The secondary is wound 
in a great many instead of a few sections each separately in¬ 
sulated from adjacent sections. An effort has been made to 
give the secondary the most advantageous size and shape as 
regards the magnetic field of the primary, and for this pur¬ 
pose the secondary coil is more than a foot shorter than the 
iron core, which latter, therefore, projects to an unusual ex¬ 
tent beyond the ends of the secondary. The great saving of 
wire in the secondary is effected largely by this disposition. 
Moreover, in the middle portion the diameter of the secondary 
coils is also reduced. 
It may be remarked that sparks exceeding 6 feet or more 
in length have been produced by others, especially by Profs. 
* Scientific American, January 5, 1901, p. 7. 
