any speed between 50 and 150 rev. per sec., is directly connected to the
drum, so that any chance of error by slipping is eliminated. The wires
leading to the primary coils of the sparking coils pass through the
explosive a meter or more apart. Wires lead from the secondary coils to
two platinum points placed a fraction of a millimeter from the periphery
of the drum. A separate circuit is provided for the firing lines.

In making a test, the separate cartridges, with the paper trimmed from
the ends, are placed, end to end, in the sheet-iron tube; the drum is
given the desired peripheral speed, and the charge is exploded. The
usual length between the points in the tube is 1 m., and the time
required for the detonation of a charge of that length is shown by the
distance between the beginning of two rows of dots on the drum made by
the sparks from the secondary coil circuits, the dots starting the
instant the primary circuits are broken by the detonation. At one end of
the drum are gear teeth, 1 mm. apart on centers, which can be made to
engage a worm revolving a pointer in front of a dial graduated to
hundredths; by means of this and a filar eyepiece, the distance between
the start of the two rows of spark dots on the drum can be measured
accurately to 0.01 mm. As the drum is 500 mm. in circumference, and its
normal speed is 86 rev. per sec., it is theoretically possible to
measure time to one four-millionth of a second, though with a cartridge
1 m. long, such refinement has not been found necessary.

The use of small lead blocks affords another means of determining the
rate of detonation or quickness of an explosive. Each block (a cylinder,
2½ in. long and 1½ in. in diameter) is enclosed in a piece of paper so
that a shell is formed above the block, in which to place the charge. A
small steel disk of the same diameter as the block is first placed in
the shell on top of the block, then the charge with a detonator is