The Polychaeta contrast with the Oligochaeta by the great variety of outward form and by the frequency of specialization of different regions of the body.
Thecurrent being alternately transmitted and shut off, as a hole passes on and off the aperture of the tube or bellows, causes a vibratory motion of the air, whose frequency depends on the number of times per second that a perforation passes the mouth of the tube.
The addiction of the Franks in later centuries to the chase is evidenced by the frequency with which not only the laity but also the clergy were warned by provincial councils against expending so much of their time and money on hounds, hawks and falcons; and we have similar proof with regard to the habits of other Teutonic nations subsequent to the introduction of Christianity.
Quantities can be learned from experience, and from watching individual cases; frequency varies within very wide limits, from reptiles which at most may feed once a week and fast for long periods, to the smaller insectivorous birds which require to be fed every two or three hours, and which in the winter dark of northern latitudes must be lighted up once or twice in the night to have the opportunity of feeding.
In circuits possessing high resistance and capacity and low inductance per mile, telephonic currents are rapidly attenuated, and the higher the frequency the more rapid is the attenuation.
Hence, when the coil at one fixed station was in action it generated high frequency alternating currents, which were propagated across the air gap between the ordinary telegraph wires and the metallic surfaces attached to one secondary terminal of the induction coil, and conveyed along the ordinary telegraph wires between station and moving train.
A good ammeter should comply with the following qualifications: - (i) its readings should be the same for the same current whether reached by increasing from a lower current or decreasing from a higher current; (2) if used for alternating currents its indications should not vary with the frequency within the range of frequency for which it is likely to be used; (3) it should not be disturbed by external magnetic fields; (4) the scale divisions should, if possible, be equal in length and there should be no dead part in the scale.
Leyden jars are now much employed for the production of the high frequency electric currents used in wireless telegraphy (see Telegraphy, Wireless).
It has sometimes been claimed that Edison's proposed elevated plates anticipated the subsequent invention by Marconi of the aerial wire or antenna, but it is particularly to be noticed that Edison employed no spark gap or means for creating electrical high frequency oscillations in these wires.
Thermal ammeters recommend themselves for the following reasons: (I) the same instrument can be used for continuous currents and for alternating currents of low frequency; (2) there is no temperature correction; (3) if used with alternating currents no correction is necessary for frequency, unless that frequency is very high.