Time of Relative Resistance Relative Resistance Electrification. at 0? C (32? F.) at 24? C (75? F.) 1 minute 100 5.51 2 " 127.9 6.
5 " 163.1 6.66 10 " 190.9 6.94 20 " 230.8 7.38 30 " 250.6 7.44 60 " 290.4 7.6 90 " 318.3 7.66
In cable testing one minute is generally taken as the time of electrification.
Pressure increases the resistance by the formula Rp=R (1+ .00327 P) in which Rp is the resistance at pressure p--R resistance at atmospheric pressure--p pressure in atmospheres. Thus in the ocean at a depth of 4,000 meters (2.4855 miles), the resistance is more than doubled. The longer the pressure is applied, the greater is the resistance.
The specific inductive capacity of gutta percha is 4.2.
Good gutta percha should not break when struck with a hammer, should recover its shape slowly, and it should support much more than 300 times its own weight.
Gyrostatic Action of Armatures.
Owing to gyrostatic action a rotating armature resists any change of direction of its axis. On ships and in railway motors which have to turn curves this action occurs. A 148 lb. armature running at 1,300 revolutions per minute may press with 30 lbs. on each journal as the ship rolls through an angle of 20? in 16 seconds.
H.
(a) The symbol for the horizontal component of the earth"s magnetization.
(b) The symbol for the intensity of a magnetizing force or field. The symbol H, as it is generally used, may mean either the number of dynes which act upon a unit pole, or the number of lines of force per centimeter.
(c) The symbol for the unit of self-induction.
Hair, Removal of, by Electrolysis.
A method of depilation by destruction of individual hair follicles by electrolysis.
A fine platinum electrode is thrust into a hair follicle. It is the negative electrode. The positive electrode is in contact with the body of the person under treatment; it is often a sponge electrode simply held in his hand. A current of two to four milliamperes from an E. M. F.
of 15 to 20 volts, is pa.s.sed. This destroys the follicle, the hair is removed and never grows again. A gradual increase of current is advised for the face. As only one hair is removed at once, but a small number are taken out at a sitting.
284 STANDARD ELECTRICAL DICTIONARY.
Haldat"s Figures.
With a pole of a strong bar magnet, used like a pencil, imaginary figures are drawn upon a hard steel plate, such as a saw-blade. The pattern is gone over several times. By dusting iron filings on a sheet of paper laid over the steel plate, while horizontal, very complicated magnetic figures are produced.
Hall"s Experiment.
A cross of thin metal, such as gold leaf, is secured upon a pane of gla.s.s. To two opposite arms a battery is connected in circuit with them.
To the other two arms a galvanometer is connected in circuit. If the cross is put into a field of force whose lines are perpendicular thereto, the galvanometer will disclose a constant current. The current is pushed, as it were, into the galvanometer circuit. Other metals have been used with similar results. They must be thin or the experiment fails. If the arm receiving the battery current is horizontal, and if it flows from left to right, and if the lines of force go from downward through the cross, the current in the galvanometer circuit will flow from the observer through the other arms of the cross, if the cross is of gold, silver, platinum or tin, and the reverse if of iron. The experiment has indicated a possible way of reaching the velocity of electricity in absolute measure.
Hall Effect.
The effect observed in Hall"s experiment, q. v.
Hall Effect, Real.
A transverse electro-motive force in a conductor through which a current is pa.s.sing produced by a magnetic field.
Hall Effect, Spurious.
A spurious electro-motive force produced in a conductor, through which a current is pa.s.sing by changes in conductivity of the conductor brought about by a magnetic field.
Hanger Board.
A board containing two terminals, a suspending hook, and a switch, so that an arc lamp can be introduced into a circuit thereby, or can be removed as desired.
Harmonic Receiver.
A receiver containing a vibrating reed, acted on by an electro-magnet.
Such a reed answers only to impulses tuned to its own pitch. If such are received from the magnet it will vibrate. Impulses not in tune with it will not affect it. (See Telegraph, Harmonic.)
Head Bath, Electric.
A fanciful name for an electro-medical treatment of the head. The patient is insulated by an insulating stool or otherwise. His person is connected with one terminal of an influence machine. An insulated metallic circle, with points of metal projecting inward or downward, is placed about the head. The circle is connected with the other pole of the machine. On working it a silent or brush discharge with air convection streams occurs between the patient"s head and the circle of points.
285 STANDARD ELECTRICAL DICTIONARY.
Head-light, Electric.
An electric head-light for locomotives has been experimented with. It includes the parabolic reflection of the regular light with an arc-lamp in place of the oil lamp. An incandescent lamp may be used in the same place, but has no great advantage over oil as regards illuminating power.
Heat.
A form of kinetic energy, due to a confused oscillatory movement of the molecules of a body. Heat is not motion, as a heated body does not change its place; it is not momentum, but it is the energy of motion. If the quant.i.ty of molecular motion is doubled the momentum of the molecules is also doubled, but the molecular mechanical energy or heat is quadrupled.
As a form of energy it is measured by thermal units. The calorie is the most important, and unfortunately the same term applies to two units, the gram-degree C. and the kilogram-degree C. (See Calorie.) Calories are determined by a calorimeter, q. v.
Independent of quant.i.ty of heat a body may be hotter or colder.
Thermometers are used to determine its temperature.
Heat is transmitted by conduction, a body conducting it slowly for some distance through its own substance. Bodies vary greatly in their conductivity for heat. It is also transmitted by convection of gases or liquids, when the heated molecules traveling through the ma.s.s impart their heat to other parts. Finally it is transmitted by ether waves with probably the speed of light. This mode of transmission and the phenomena of it were attributed to radiant heat. As a scientific term this is now dropped by many scientists. This practice very properly restricts the term "heat" to kinetic molecular motion.
The mechanical equivalent of heat is the number of units of work which the energy of one unit quant.i.ty of heat represents. (See Equivalents, Mechanical and Physical.)
Heat, Atomic.
The product of the specific heat of an element by its atomic weight. The product is approximately the same for all the elements, and varies as determined between 5.39 and 6.87. The variations are by some attributed princ.i.p.ally to imperfection of the work in determining them. The atomic heat represents the number of gram calories required to raise the temperature of a gram atom (a number of grams equal numerically to the atomic weight) one degree centigrade.
286 STANDARD ELECTRICAL DICTIONARY.
Heat, Electric.
This term has been given to the heat produced by the pa.s.sage of a current of electricity through a conductor. It is really electrically produced heat, the above term being a misnomer.