Radio Digest (Apr 1925-Jan 1926)

June 6, 1925 RADIO D I G E S T— Illustrated 19 A. B. C. Course in Radio Fundamentals Chapter XI — Electrical Measuring Instruments (Part 2) By David Penn Moreton THE fundamental principle of the electrodynamometer type of ammeter is shown in figure 53. The current to be measured is sent through two coils, Ci, C2, connected in series. One of the coils, Ci, is mounted on bearings so that it may turn inside of the other coil, Cj. Two coil springs, SS, control the position of the movable coil and at the same time provide an electrical connection to it. Now, when there is a current in the ammeter, the following conditions will exist. There will be a magnetic field produced inside of the stationary coil whose strength will vary directly as the value of the current in the coil. As a result of there being a current in the movable coil, and since it is in a magnetic field, there will be a force acting on the coil which will cause it to rotate against the force due to the springs. The coil will come to rest in a position such that the force due to the action of the current will be exactly equal to the force due to the springs. The deflecting force, or the force which moves the coil from its zero position, varies in value with the value of the current in the movable coil and the strength of the magnetic field inside the stationary coil, which in turn varies in value with the current in this coil, which is the same as the current in the movable coil, or, the deflecting force will vary in value with the product of the current in the stationary coil and the current in the movable coil or, the current squared. The scale of the instruments, however, are usually marked to read the current directly. Direction of Current No Worry An instrument of this kind will always deflect in the same direction when there is a current sent though it, regardless of the direction of the current. When the current is reversed in direction, there is a reversal of direction of the magnetic field within the stationary coil and at the same time there is a reversal of current in the movable coil and this double reversal results in the force acting on the movable coil remaining unchanged in direction. When an Instrument of this kind is used in measuring direct current, the deflection of the pointer may be different with the current in one direction than it is with the current in the reverse direction, although the value of the current is exactly the same in both cases. This difference in deflection is due to the action of any magnetic field, other than the magnetizing action of the current in the instruments, which may exist in the region in which the instrument is located. Such magnetic fields usually produce a different effect with the current in one direction than they produce with the current in the reverse direction, which accounts for the difference in deflection. When an alternating current is being measured, the effect of any outside magnetic fields is to increase the deflection for one alternation of the current and to decrease the deflection an equal amount for an alternation in the opposite direction so that the actual or average deflection is independent of the effect of the outside magnetic fields. Of course, when the outside magnetic A lidio Frequency Trantformrr Quality Parts Matched for Perfect Teamwork Your "net" hook-up needs first quality parts — f>cr/ectljp matched— to give you real radio. Every Federal Standard Radio Part is designed, made, matched and guaranteed by Federal. That is why you find Federal parts in all the better hook-ups — that is why you should insist on Federal parts when purchasing. FEDERAL TELEPHONE MANUFACTURING CORP Buffalo. N. ^ * Standard RADIOProduct9 fields vary in value and direction at the same frequency as the current in the instrument, then their effect is not neutralized by the reversal of current and they cause an error in the indication of the instrument. The fundamental principle upon which another type of ammeter, known as the "magnetic vane" or "soft iron" type, operates, may be explained by reference to figure 54. A coil C carries the current to be measured, and this coil is mounted in an inclined position. A piece of soft sheet iron I is mounted on a shaft T, which is in turn mounted in a vertical position inside the coil C. A sping S holds the pointer P, which is attached to the shaft T, in its zero position. The piece of iron I is mounted on the shaft T with its planes making an angle of something like 45 degrees to the shaft and almost parallel to the plane of the coil, when the pointer is in its zero position. Now when there is a current in the coil, a magnetic field will be produced within the coil and the piece of soft iron will be magnetized, and as a result there will be a force acting on the piece of iron tending to turn it so that its longest dimension is parallel to the axis of the coil. The value of this force will, of course, depend upon the value of the current in the coil, increasing with an increase in current and decreasing with a decrease in the current. The direction of the current in the coil has nothing to do with the direction of the force because the polarity of the piece of iron changes when the current is reversed and the deflecting force will have the same direction. What Happens with Direct Current When an instrument of this kind Is used to measure a direct current, the deflection may be influenced by stray magnetic fields as explained in the case of the electrodynamometer type of instruments. The magnetism in the piece of iron lags behind the current and this will cause the instrument to indicate low.er values than it should when the current in increasing, and, likewise, higher values when the current is decreasing. An ammeter is always connected in series in the circuit in which the current is to be measured, and in order that the introduction of the instrument into the circuit may produce as small an effect as possible, the resistance of the instrument should be very small in comparison to the resistance of the remainder of the circuit. A high resistance ammeter would not only produce a noticeable Figure 54 change in the total resistance of the circuit and hence the values of the current, but there would be a large loss in the high resistance ammeter in comparison to the loss that would occur in the lew resistance instruments. Use of Ammeter Shunts When small currents are to be measured the total current passes through the coil or coils of the instruments. These coils mid the electrical connections to them would be very cumbersome If thej were made large enough to handle heavy currents, and in .such cases use is made of what is called an ammeter shunt. The connection of the ammeter shunt Is (Continued on page 20) Summer Static Now overcome by Volume! Enjoy Radio All the Year Around —This Way! HOW often have you heard it said, "Well, the warm weather is coming on, I had better pul mj radii 1 he shelf. It will be of no use until Fall." There is absolute!] no way i" eliminate all static from your radio ret lion, but today there i« a waj to reduce it to a point where it ceaiei t" atmev. 1 in . new way to 01 er COtne static is l>v volume. And volume can besl '»• obtained by Using trans formers that v. ill amplify ignals to theii .•lb. Here are the Kellogg audio and radio frequency t r a 11 s f o r in e r B, Kelli Transformers give volume with clarity of tone. 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