Questions And Answers: Definitions - Pupin And Tesla Coils

Saturday, October 1, 1910
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Popular Electricity October 1910 Use of this department is free to readers of Popular Electricity, but atten- tion will not be given to questions which do not comply with the fo1loW» ing rules: All questions must be written in the form of a letter addressed to the Questions and Answers Department and containing nothing for the other departments of the magazine; two»cent stamp must be enclosed for answer by mail, for space will not permit of printing all answers; the full name and address of the writer must be given. Dennitions; Pupin and Tesla Coils; Mercury Vapor Lamps Questions.--(A) What is the difference between a spark coil and an open core transformer? (B) (I) What is inductance? (2) Impedance? (3) Re- luctance? (4) Admittance? (C) What is a Tesla coil? (D) What is a Pupin coil? (E) Are mer- cury vapor lamps made to work on both no volts alternating and direct current? How many watts per candle-pover do they consume E--M. S., Fort Worth, 'I`exas.f Answers.-(A) A spark coil is built along the same lines as an open core transformer, and is a transformer. The ratio of trans< formation is usually much lower in the ordi~ nary transformer than in the spark coil. In the latter a make-and-break contact with acondenser connected across this contact greatly increases the length, thickness, and brilliancy of the spark which is intermittent, while in the transformer a steady tlow of current from the secondary is usual. (B) (r) In general by inductance we mean that inexplicable formation of lines about a wire or coil of wire which a magnetic needle tells us are there when a current Hows in the wire or coil. These lines represent energy and exercise an inductive effect on adjacent \vires. (2) Circuits always have resistance as well as inductance. The combined effect of the two is called impedance and is ex- pressed by the formula: Impedance in ohms= VR’ + (2X3.I4I6X'fXL)7 in which R=resistance of the circuit f =frequency L = inductance. (3) The term reluctance applies to the re- sistance oifered by a magnetic path to the tlow of lines of force. For example, the reluctance of wrought iron is less than that of cast iron. the former reouirinr less amoere- turns to produce a given flux. (4) If I represents the impedance of a circuit in I ohms,- is its admittance. Used in fig- I uring problems regarding A. C. circuits in parallel, into which calculations resistance, inductance and capacity enter. (C) /\ form of induction coil built to give high potentials and high frequencies. This coil and the method of operation is described fully in the ]anuary, IQO9, issue. (D) In a long telephone cable the speale ing current is distorted, due to the static capacity of the conductor, and is also blurred it the frequency is high. To balance this capacity and get rid oi the blurred sound effect, Professor Pupin places an ordinary coil of wire containing a laminated iron core in each circuit at intervals oneeighth of a mile apart. By the introduction of this inductance the talking current is modified so that the proper sounds are registered at the receiving end. (E) Mercury vapor lamps are made for either direct or alternating current, but the same lamp can be used only upon the kind of current for which it is designed. Lamps requiring rio volts take 3.5 amperes or 385 watts, the candle-power ranging according to type from 3oo to 8oo.