Radio Broadcast (Nov 1926-Apr 1927)

306 RADIO BROADCAST ADVERTISER SUPEREM/SS/ON ■■mho TUBES arethe most perfectly balanced tubes produced * X 200 A Super-Sensitive Power Detector Other Type3 201 A 5 volt detector 'amplifier X ii2 5 " power amplifier detector 'amplifier power amplifier detector-amplifier 199 3 X 120 3 12 1* and SPEED FULL WAVE GAS FILLED RECTIFIER FOR USE IN STANDARD "B" BATTERY ELIMINATORS A FEW FRANCHISES STILL OPEN FOR FIRST CLASS DISTRIRUTORS Cable Supply Company INC. Executive Offices 31 Union Square New York inators, shooting trouble, and so on. Since the field of application of the milliammeter is so large, several different ranges are desirable. Manufacturers supply voltmeters with high and low ranges, but multi-range milliammeters are not available, and the demands upon one's purse usually preclude the luxury of several meters. In electrical laboratories where large currents are often measured, ammeters of low currentcarrying capacity are adapted to this purpose by means of an "external shunt." This is nothing more than a resistance unit of such a value as to bypass a known portion of the current around the meter. This practice may be conveniently adopted by the home experimenter in making external multipliers for his milliammeter. As an example, suppose a current estimated at about 50 milliamperes is about to be measured with a 0-10 mil. meter. If the meter resistance is 4 ohms, a shunt resistance of 1 ohm connected across its terminals will cause the current to divide inversely proportional to the resistances, 40 mils, going through the shunt, and 10 mils, going through the meter and registering on the scale as such. The meter with its shunt is then said to have a scale factor of 5, since the reading must be multiplied by 5 to obtain the value of the total current flowing. Scale multipliers are easily made without the necessity of measuring either their own resistance or that of the milliammeter. Connect the meter in series with any high resistance unit and sufficient B battery voltage to give about a full scale deflection of the needle. A No. 10 Bradleyohm Solder - 1 ~ Sealing ! ; >~ — ' wax FIG. 2 factor to obtain the value of the total measured current. Homer S. Davis, Memphis, Tennessee. A Handy Reference Book THOSE who have been subscribers to Radio Broadcast from the first issue have by this time acquired a rather large fund of useful information. It is a great aid to the experimenter if he has this information instantly available. The Reference Book described here, will eliminate the necessity of looking through perhaps a dozen volumes for some elusive article. With a razor blade, cut the "Contents" page from each volume. Though Radio Broadcast -Clamp Twelve Contents" pages J is ideal for this purpose, as it may be adjusted to give exactly full-scale deflection. The resistance wire to be shunted across the meter may be obtained from an old rheostat, preferably of 30ohm size. The "dime store" variety is satisfactory. After removing the winding from the frame, cut off about two dozen turns from the end, including the strip. When this wire is connected across the two terminals of the meter, less deflection of the needle will be noted. The length of the resistance wire should be gradually reduced until the needle shows the desired fraction of the original reading, as one-half, or onefifth, etc. For these trials the ends of the wire should be well tinned as for soldering; if this is not done until last, the final reading will be spoiled. It is best to solder the terminal lugs in place just before the correct length is reached; further reduction may then be accurately accomplished by carefully tinning a small portion of the wire at a time. This should be accurately done, as any error here will multiply as the scale factor increases. Factors of 10 or more are hardly practicable; in such cases it is better to take the meter with its shunt to a neighboring transmitting amateur, who will gladly check it against his high-range meter. The unit may be made rigid by coating it with sealing wax, as shown in the diagram, Fig. 2. Place it in position on the meter terminals and, with the aid of a small alcohol blow torch, work the melted wax around it. The scale factor may then be marked on the wax. In using, the meter reading should be multiplied by this Examined and approved by Radio Broadcast -Je k. Glue ' Binding strip FIG. 3 has been issued in various sizes, it will be found that each "Contents" page can be cut to about seven by nine and one-half inches without destroying the table desired. All the sheets should be cut to this size. Take the sheets of the first twelve issues and lay the last of these on a table, face up. Apply glue to a strip one half inch wide up the left edge. Then lay the second last sheet on this, and apply glue as before. Do this with the ten remaining sheets. Then bind the folios with heavy paper, as shown in Fig. 3, and clamp the booklet in a vise till the glue hardens. Then remove it and drill two holes through the bound edge as shown in Fig. 4. When folios for each year have been built up in this manner, place them one on top of the other and trim them with a razor blade to a uniform size. Make a cover of black oilcloth, faced with cardboard, and cut holes in it corresponding to those through the bound edges of the folios. (See Fig. 4.) Assemble the book as shown, by means of Terminal Nut Black oilcloth ccvei FIG. 4 brass bolts and battery terminals. This method of assembly permits the addition of folios from time to time, but eliminates the possibility of tearing the sheets from the book. Ronald \V. Klinck, Vancouver, British Columbia.