The amplitude of the signals can be varied in several ways, either by a shunt across the electromagnet, or by altering the tension of the controlling springs or by altering the air gap between electromagnets and armatures.
By means of a " magnetic shunt " Brown succeeded in increasing the working speed of long submarine cables to the extent of To to 15 per cent.
The magnetic shunt (which is connected Magnetic across the receiving instrument) consists of a low resist- shunt.
The use of the iron core renders it possible to produce a high inductive effect with a low resistance coil.
The explanation of the action of the shunt is that all slowly varying currents affect the coil of the receiving instrument and its shunt in inverse proportion to their respective resistances; whereas with the comparatively rapid variations of current used in signalling the coil is forced at the beginning of each element of A v
A signal to take more, and at the end of the element less of the total arrival current from the cable than would traverse it if the shunt were non-inductive.
This consists of a low resistance coil of copper wire enclosed in a laminated iron circuit similar to the magnetic shunt already de Magnetic scribed.
The action of this bridge resembles the magnetic shunt in its effect on the received signals, as the direction of the winding is the same throughout its length, and thus the full inductive action is produced for curbing purposes.
In instruments for larger currents the main current passes through a metallic strip acting as a bye-pass or shunt, and to the ends of this shunt are attached the ends of the working wire.
This shunt is generally a strip of platinoid or constantin, and the working wire itself is of the same metal.
There is therefore a certain ratio in which any current passing through the ammeter is divided between the shunt and the working wire.
It is also necessary to notice that shunt instruments cannot be used for high frequencies, as then the relative inductance of the shunt and wire becomes important and affects the ratio in which the current is divided, whereas for low frequency currents the inductance is unimportant.
In the case of ammeters intended for very small currents, the whole current can be sent through the coil, but for larger currents it is necessary to provide in the instrument a shunt which carries the main current, the movable coil being connected to the ends of this shunt so that it takes a definite small fraction of the current passed through the instrument.
In this last case the shunt need not be contained in the instrument itself but may be at a considerable distance, wires being brought from the shunt which carries the main current to the movable coil ammeter itself, which performs the function simply of an indicator, 3.
This rod was connected with the negative pole of the generator, and was suspended from one arm of a balance-beam, while from the other end of the beam was suspended a vertical hollow iron cylinder, which could be moved into or out of a wire coil or solenoid joined as a shunt across the two carbon rods of the furnace.
Any change in the resistance of the arc, either by lengthening, due to the sinking of the charge in the crucible, or by the burning of the carbon, affected the proportion of current flowing in the two shunt circuits, and so altered the position of the iron cylinder in the solenoid that the length of arc was, within limits, automatically regulated.
The shunt coil Sh and the series coil Se are connected together at one point, and the remaining terminals of the dynamo and shunt coil must be connected to a " good earth," which is generally the gas or water pipes w of the building.
On setting the dynamo in operation, a current passes through the shunt coil of the ohmmeter proportional to the voltage of the dynamo, and, if there is any sensible leakage through the insulator to earth, at the same time another current passes through the series coil proportional to the conductivity of the insulation of the wiring under the electromotive force used.
The two coils, the shunt and the series coil, then produce two magnetic fields, with their lines of force at right angles to one another.
The tangent of the angle of deflection 0 of this needle measured from its position, when the shunt coil is disconnected, is equal to the ratio of the voltage of the dynamo to the current through the insulator.
The other is a shunt across the terminals of the armature.
The meaning of this deflection can be interpreted as follows: If a galvanometer has a resistance R and is shunted by a shunt of resistance S, and the shunted galvanometer is placed in series with a large resistance R' of the order of a megohm, and if the same w w Se J FIG.
It is possible so to arrange the value of the shunt and of the high resistance R' that the same or nearly the same deflection of the galvanometer is obtained as when it is used in series with the battery and the insulation-resistance.
It consisted of a glass vessel, containing a solution of sulphate of zinc, in which were placed two plates of pure amalgamated zinc. These plates were connected by means of a german-silver shunt, their size and the distance between them being so adjusted that about ii 0 - 0 - part of the current passing through the meter travelled through the electrolytic cell and -j o i j of the current passed through the shunt.
To prevent temperature from affecting the shunt ratio, Edison joined in series with the electrolytic cell a copper coil the resistance of which increased with a rise of temperature by the same amount that the electrolyte decreased.
The current to be measured passes through the fixed field-coils, whilst through the armature passes a shunt current obtained by connecting the brushes across the supply mains through a constant resistance.
In order to overcome the friction of the train the field-coils are wound with an auxiliary shunt coil which supplies a driving force sufficient to overcome the friction of the counting train.
By the use of a permanent magnet instead of a shunt coil as the bob of one pendulum, the meter can be made up as an ampere-hour meter.
In proximity to the upper side of the disk is placed a coil of wire having an iron core, which is a shunt coil, the ends of the coil being connected to the terminals of the supply mains.
The current in the shunt coil lags 90 degrees behind the impressed electromotive force of the circuit to be measured; hence if the main current is in step with the potential difference of the terminals of the supply mains, which is the case when the supply is given wholly to electric lamps, then the field due to the main coil differs from that due to the shunt coil by 90 degrees.
Since the eddy currents induced in the disk are 90 degrees in phase behind the inducing field, the eddy currents produced by the main coil are in step with the magnetic field due to the shunt coil, and hence the disk is driven round by the revolution due to the action of the shunt coil upon the induced currents in the disk.
A common source of trouble is the short circuiting of the shunt coils owing to the shellaced cotton covering of the wire becoming moist.
On the other hand the resistance of the shunt coil absorbs energy which generally varies from i to 3 watts and is a loss either to the consumer or to the supply company, according to the manner in which the shunt coil is connected.
In those meters which are compounded - that is, have a shunt coil wound on the field magnets to compensate for the friction of the train - it is important to notice whether the meter will operate or continue operating when there is no current in the series coil, since a meter which "runs on the shunt" runs up a debt against the consumer for which it gives no corresponding advantage.
Again the shunt circuit must have practically zero inductance and the series or current coil must be wound or constructed with stranded copper wire, each strand being silk Covered, to prevent the production of eddy currents in the mass of the conductor.