John Ambrose Fleming

“ The rotatory transformer thus absorbs continuous current power and emits it in a periodic form; accordingly, there is a continual storage and emission of energy by the armature, and therefore its kinetic energy is periodically varying during the phase. The armature is also creating a back-electromotive force which acts at some instants against the voltage driving the current into the armature and at others is creating an electromotive force that assists the external impressed voltage in driving a current through the alternating current side. ”

“ In large transformers it is usual to employ some means of producing a current of air through the core to ventilate it. In these, called air-blast transformers, apertures are left in the core by means of which the cooling air can reach the interior portions. This air is driven through the core by a fan actuated by an alternating current motor, which does not, however, take up power to a greater extent than about 1/4or 1/10% of the full output of the transformer, and well repays the outlay. ”

“ A low power factor always implies a magnetic circuit of large reluctance.
The operation of the alternating current is then as follows: the periodic magnetizing force of the primary circuit creates a periodic magnetic flux in the core, and this being linked with the primary circuit creates by its variation what is called the back electromotive force in the primary circuit. The variation of the particular portion of this periodic flux, linked with the secondary circuit, originates in this last a periodic electromotive force. ”

“ If the primary electromotive force curve has the form of a high peak, or runs up suddenly to a large maximum value, the flux-density curve will be more square-shouldered than when the voltage curve has a lower form factor. ”

“ No test of a transformer is complete which does not comprise some investigation of the “ageing” of the core. The slow changes which take place in the hysteretic quality of iron when heated, in the case of certain brands, give rise to a time-increase in iron core loss. Hence a transformer which has a core loss, say, of 300 watts when new, may, Ageing. unless the iron is well chosen, have its core loss increased from 50 to 300% by a few months’ use. ”

“ The designer of a transformer core has in view, first, economy in metal, so that there may be no waste fragments, and second, a mode of construction that facilitates the winding of the wire circuits. These consist of coils of cotton-covered copper wire which are wound on formers and baked after being well saturated with shellac varnish: The primary and secondary circuits are sometimes formed of separate bobbins which are sandwiched in between each other ”

“ Reference must be made to the continuous current transformer. The conversion of a continuous current supplied, say, at 100 volts, into one having an electromotive force of 10 volts, can of course be achieved by coupling together on the same bedplate a suitable electric motor and a dynamo. The combination is called a motor-dynamo set, and each Continuous Current Transformers. machine preserves its own identity and peculiarity. The same result may, however, be accomplished by winding two separate armature circuits on one iron core, and furnishing each with its own commutator. ”

“ This unilateral conductivity is dependent on a certain voltage or potential difference between the plates not being exceeded, but within these limits a plate of carbon and aluminium placed in a solution, say of hydro-sodic phosphate, acts as an electrical valve, allowing current to pass in one direction but not in another. An examination of the aluminium plate after it has been so used shows that its appearance has changed and that its surface is covered by a thin film, the thickness of which varies with the electrolyte and the time of formation. ”

“ The electrolytic rectifier is based upon the fact that if plates of aluminium and carbon are placed in an electrolyte, say a solution of alum or dilute acids which yield oxygen on electrolysis, it is found that a current can be sent through the liquid from the carbon to the aluminium, but that great counter electromotive force is created to a current in the opposite direction. ”

“ A rotatory or rotary transformer may consist of one machine, or of two separate machines, adapted for converting a single-phase alternating current into a poly phase current, or a poly phase current into a continuous current, or a continuous current into an alternating current. ”

“ It is impossible that the unilateral conductivity can be due to a true electrolytic polarization, because we know of no polarization of this latter kind which exceeds three volts, and the film can be made to resist the flow of a current under an electromotive force of 140 to 200 volts. The resistance of this film has been measured and found to be very high, so high as to be practically an insulation. ”

“ In this manner he secures good ventilation and a minimum expenditure in copper and iron, as well as the possibility of insulating the two copper circuits well from each other and from the core. An important matter is the cooling of the core. This may be effected either by ordinary radiation, or by a forced draught of air made by a fan or else by immersing the transformer in oil, the oil being kept cool by pipes through which cold water circulates immersed in it. ”

“ Transformers have even been built to give secondary voltages of half a million volts capable of giving a 14 in. spark in air. These machines, however, must be regarded as more physical laboratory instruments than appliances for technical work. ”

“ The cell is filled with. a. saturated solution of ammonium phosphate, and a, nonconducting shielding tube can be slid over the aluminium electrode to alter the exposed area.
The valve is shown in section in fig. 12, and the 4 cells are arranged in a Wheatstone’s Bridge fashion, as shown in fig. 13. A and A1 are the terminals to which the alternating current is supplied, C and C1 the terminals from which the continuous current is drawn off. ”