between rather wide limits,
as to the rate at which it vibrates.

As a rule harmonic party-line systems are limited to four stations on
a line. The frequencies employed are usually 16-2/3, 33-1/3, 50, and
66-2/3 cycles per second, this corresponding to 1,000, 2,000, 3,000,
and 4,000 cycles per minute. The reason why this particular set of
frequencies was chosen is that they represent approximately the range
of desirable frequencies, and that the first ringing-current machines
in such systems were made by mounting the armatures of four different
generators on a single shaft, these having, respectively, two poles,
four poles, six poles, and eight poles each. The two-pole generator
gave one cycle per revolution, the four-pole two, the six-pole three,
and the eight-pole four, so that by running the shaft of the machine
at exactly 1,000 revolutions per minute the frequencies before
mentioned were attained. This range of frequencies having proved
about right for general practice and the early ringers all having been
attuned so as to operate on this basis, the practice of adhering to
these numbers of vibrations has been kept up with one exception by all
the manufacturers who make this type of ringer.

_Tuning._ The process of adjusting the armature of a ringer to a
certain rate of vibration is called tuning, and it is customary to
refer to a ringer as being tuned to a certain rate of vibration, just
as it is customary to refer to a violin string as being tuned to a
certain pitch or rate of vibration.

The physical difference between the ringers of the various frequencies
consists mainly in the size of the weights at the end of the vibrating
reed, that is, of the weights which form the tapper for the bell. The
low-frequency ringers have the largest weights and the high-frequency
the smallest, of course. The ringers are roughly tuned to the desired
frequencies by merely placing on the tapper rod the desired weight and
then a more refined tuning is given them by slightly altering the
positions of the weights on the tapper rod. To make the reed have a
slightly lower natural rate of vibration, the weight is moved further
from the stationary end of the reed, while to give it a slightly
higher natural rate of vibration the weight is moved toward the
stationary. In this way very nice adjustments may be made, and the aim
of the various factories manufacturing these bells is to make the
adjustment permanent so that it will never have