[1] _Experimental Researches in Electricity_, series ii. Sec. 6, pars.
256, 259-260, and series xxviii. Sec. 34.
[2] _Ibid._ series i. Sec. 4, pars. 84-90.
[3] "On the Physical Lines of Magnetic Force," _Phil. Mag._, June
1852.
[4] Faraday, _Exp. Res._ series xxviii. Sec. 34, pars. 3104, 3114-3115.
[5] _Id._, ib. series i. Sec. 4, pars. 114-119.
[6] _Id._, ib. series ii. Sec. 6, pars. 211, 213; series xxviii. Sec. 34,
par. 3152.
[7] Invented by Nikola Tesla (_Elec. Eng._ vol. xiii. p. 83. Cf.
Brit. Pat. Spec. Nos. 2801 and 2812, 1894). Several early inventors,
e.g. Salvatore dal Negro in 1832 (_Phil. Mag._ third series, vol. i.
p. 45), adopted reciprocating or oscillatory motion, and this was
again tried by Edison in 1878.
[8] The advantage to be obtained by making the poles closely embrace
the armature core was first realized by Dr Werner von Siemens in his
"shuttle-wound" armature (Brit. Pat. No. 2107, 1856).
[9] _Nuovo Cimento_ (1865), 19, 378.
[10] Brit. Pat. No. 1668 (1870); _Comptes rendus_ (1871), 73, 175.
[11] _Ann. Chim. Phys._ l. 322.
[12] Ibid. li. 76. Since in H. Pixii's machine the armature was
stationary, while both magnet and commutator rotated, four brushes
were used, and the arrangement was not so simple as the split-ring
described above, although the result was the same. J. Saxton's
machine (1833) and E.M. Clarke's machine (1835, see Sturgeon's
_Annals of Electricity_, i. 145) were similar to one another in that
a unidirected current was obtained by utilizing every alternate
half-wave of E.M.F., but the former still employed mercury collecting
cups, while the latter employed metal brushes. W. Sturgeon in 1835
followed Pixii in utilizing the entire wave of E.M.F., and abandoned
the mercury cups in favour of metal brushes pressing on four
semicircular disks (_Scientific Researches_, p. 252). The simple
split-ring is described by Sir C. Wheatstone and Sir W.F. Cooke in
their Patent No. 8345 (1840).
[13] By the "leading" side of the tooth or of an armature coil or
sector is to be understood that side which first enters under a pole
after passing through the interpolar gap, and the edge of the pole
under which it enters is here termed the "leading" edge as opposed to
the "trailing" edge or corner from under which a tooth or coil
emerges into the gap between the poles; cf. fig. 30, where the
leading and trailing pole-corners are marked ll and tt.
[14] Such was the arrangement of Wheatstone's machine (Brit. Pat. No.
9022) of 1841, which was the first to give a more nearly "continuous"
current, the number of sections and split-rings being five.
[15] Its development from the split-ring was due to Pacinotti and
Gramme (Brit. Pat. No. 1668, 1870) in connexion with their ring
armatures.
[16] And extended by G. Kapp, "On Modern Continuous-Current
Dynamo-Electric Machines," _Proc. Inst. C.E._ vol. lxxxiii. p. 136.
[17] Drs J. and E. Hopkinson, "Dynamo-Electric Machinery," Phil.
Trans., May 6, 1886; this was further expanded in a second paper on
"Dynamo-Electric Machinery," _Proc. Roy. Soc._, Feb. 15, 1892, and
both are reprinted in _Original Papers on Dynamo-Machinery and Allied
Subjects_.
[18] _Exp. Res._, series i. Sec. 4, par. 111. In 1845 Wheatstone and
Cooke patented the use of "voltaic" magnets in place of permanent
magnets (No. 10,655).
[19] Between Moutiers and Lyons, a distance of 115 m., energy is
transmitted on the Thury direct-current system at a maximum pressure
of 60,000 volts. Four groups of machines in series are employed, each
group consisting of four machines in series; the rated output of each
component machine is 75 amperes at 3900 volts or 400 h.p. A water
turbine drives two pairs of such machines through an insulating
coupling, and the sub-base of each pair of machines is separately
insulated from earth, the foundation being also of special insulating
materials.
[20] For experiments on high-frequency currents, Nikola Tesla
constructed an alternator having 384 poles and giving a frequency of
about 10,000 (_Journ. Inst. Elec. Eng._ 1892, 21, p. 82). The
opposite extreme is found in alternators directly coupled to the
Parsons steam-turbine, in which, with a speed of 3000 revs. per min.,
only two poles are required to give a frequency of 50. By a
combination of a Parsons steam-turbine running at 12,000 revs. per
min. with an alternator of 140 poles a frequency of 14,000 has been
obtained (_Engineering_, 25th of August 1899). For description of an
experimental machine for 10,000 cycles per second when running at
3000 revs. per min., see _Trans. Amer. Inst. Elect. Eng._ vol. xxiii.
p. 417.
[21] As in the historical transmission of energy from Lauffen to
Frankfort (1891).
[22] In the pioneer three-phase transmission between Laufen and
Frankfort (_Electrician_, vol. xxvi. p. 637, and xxvii. p. 548), the
three-phase current was transformed up from about 55 to 8500 volts,
the distance being 110 m. A large number of installations driven by
water power are now at work, in which energy is transmitted on the
alternating-current system over distances of about 100 m. at
pressures ranging from 20,000 to 67,000 volts.
DYNAMOMETER (Gr. [Greek: dynamis], strength, and [Greek: metron], a
measure), an instrument for measuring force exerted by men, animals and
machines. The name has been applied generally to all kinds of
instruments used in the measurement of a force, as for example electric
dynamometers, but the term specially denotes apparatus used in connexion
with the measurement of work, or in the measurement of the horse-power
of engines and motors. If P represent the average value of the component
of a force in the direction of the displacement, s, of its point of
application, the product Ps measures the work done during the
displacement. When the force acts on a body free to turn about a fixed
axis only, it is convenient to express the work done by the transformed
product T[theta], where T is the average turning moment or torque acting
to produce the displacement [theta] radians. The apparatus used to
measure P or T is the dynamometer. The factors s or [theta] are observed
independently. Apparatus is added to some dynamometers by means of which
a curve showing the variations of P on a distance base is drawn
automatically, the area of the diagram representing the work done; with
others, integrating apparatus is combined, from which the work done
during a given interval may be read off directly. It is convenient to
distinguish between absorption and transmission dynamometers. In the
first kind the work done is converted into heat; in the second it is
transmitted, after measurement, for use.
_Absorption Dynamometers._--Baron Prony's dynamometer (_Ann. Chim.
Phys._ 1821, vol. 19), which has been modified in various ways,
consists in its original form of two symmetrically shaped timber beams
clamped to the engine-shaft. When these are held from turning, their
frictional resistance may be adjusted by means of nuts on the screwed
bolts which hold them together until the shaft revolves at a given
speed. To promote smoothness of action, the rubbing surfaces are
lubricated. A weight is moved along the arm of one of the beams until
it just keeps the brake steady midway between the stops which must be
provided to hold it when the weight fails to do so. The general theory
of this kind of brake is as follows:-Let F be the whole frictional
resistance, r the common radius of the rubbing surfaces, W the force
which holds the brake from turning and whose line of action is at a
perpendicular distance R from the axis of the shaft, N the revolutions
of the shaft per minute, [omega] its angular velocity in radians per
second; then, assuming that the adjustments are made so that the
engine runs steadily at a uniform speed, and that the brake is held
still, clear of the stops and without oscillation, by W, the torque T
exerted by the engine is equal to the frictional torque Fr acting at
the brake surfaces, and this is measured by the statical moment of the
weight W about the axis of revolution; that is--