1891. ] NEW YORK ACADEMY OF SCIENCES. 107 
mechanical device, actuated either by clockwork or electro- 
magnets. 
The regulation of the length of arc that shall produce the 
most intense light with the least current is confined to very 
narrow limits, and the mechanical construction of the lamp. 
that will yield satisfactory results must be most perfect. In 
many of the lamps this regulation is still wanting, and lamps 
lacking the above essentials, that are put together without re- 
gard to any electrical laws, with the hope that chance may make 
them work, produce a hissing and hammering noise that is dis- 
agreeable when silence is required, and are not to be recom- 
mended for any purpose. 
Between the better lamps there is not much to choose for or- 
dinary illumination ; but for special purposes, as in the case of 
lantern projection, some points are of vital importance, for the 
success will depend mainly upon the steadiness, good behavior, 
and excellence of the automatic feed and position of the carbons, 
together with a uniform action of the dynamos. ... At pre- 
sent arc lights are run both by continuous and by alternat- 
ing currents—t.e., in some cases the current is steadily in the 
same direction, while in others the current consists of pulses, 
alternately positive and negative, succeeding each other at the 
rate of from 200 to 300 per second. ‘The operation of nearly all 
the arc lamps in use is based upon the principle that a current 
of electricity will divide itself in direct proportion to the resist- 
ance offered by a conductor. In an arc lamp two ways are pro- 
vided in which the current can go—either through the low or 
high resistance coil of the lamp. When the current passes 
through the low-resistance coil it pulls the feed rod up and 
strikes the arc; any alteration in the resistance will change the 
direction of the current, and thus give an automatic movement 
to the feed rod. When the resistance increases, part of the cur- 
rent will pass through the high-resistance coil, which will de- 
press the feed rod and bring the carbons together. The coils 
are so wound that the resistances due to a certain separation of 
are will just balance each other, thus maintaining a steady 
light. . .. The design of the lamp depends upon the con- 
dition with which it is to be supplied with current ; that is, 
whether it is supplied by a constant potential with a variable 
current, as is furnished by the Edison Company, or low-tension 
current—that is, having a voltage of 115 to 120 volts—or whether 
it is fed from a variable potential and constant current of high 
voltage, as is used in series are lighting. This current is known 
as high potential, and the voltage runs in most cases from 1,000: 
to 3,000 volts. The objection to the latter is that to make any 
