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Page:Popular Science Monthly Volume 89.djvu/457

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Popular Science Mniithh/

��443

��A Simple Way to Construct a Ten- Ampere Shunt

AlO-AMPERE shunt which can be used in connection with an am- meter, either of the iiome-made variet\' or of a standard t>pc, can be con- structed with Httle diffiruh\-. When used in connection with the standard ammeter it is assumed that the meter is calibrated for a much lower reading than that which is desired. Through the simple addition of the shunt, the meter may be made to measure a current several times as large as that handled without the shunt.

The materials necessarj^ are as follows: strip of German silver, brass blocks, block of wood, screws for am. leads, screws to hold the line wires tight, screws to fasten brass blocks to the block of wood, and a hole for line wire.

First, saw two pieces of brass and file them smooth, so that their dimensions are i in. by ^i in. by ?4 in. Drill holes, as shown, with a No. 25 drill, making them of suitable size for screws to hold the brass block to the base. Thread the ones for wire terminals with a lo-in. by 24-in. tap.

Secure a piece of German silver, 12 mil. thick (.012 in.) and 2^s ins. long, and calculate its width by the following method. In calculating the width, the length of the German siK'er strip is 2 ins. because the ^g in. goes into the brass block, 3/16 in. in each one. The am- meter reads 50 millivolts on full scale deflection.

I millivolt = .001 volt. 50 millivolts = .05 volt; current 10 amp. E .05

R = — = =005 ohms resistance of

I 10 the German silver. Ki Ki

R= — A=—

A R where K is the resis-

ti\ity of German silver, i is the length in ft., R the resistance, and A the area. The resistivity of German silver is 20 times that of copper. 10.4 = resis- tivity of copper. 1=2 ins. = .167 ft. so 20 X 10.4 = 208 resistivity of German sih-er.

208X.167

A = = 7214.5 cu. mils.

.005

��A = 72 14. 5 X. 7834 = 5666.36 sq. mils. 5066.36

A = = .00566 in.

1,000,000 Thickness = .012 in.

A .00566 So width = =

��TH

��.012

��.471 m.

��Cut a groov'e in each brass block 3/16 in. deep and J4 •"• up from the bottom, and solder one end of the German silver strip in each block, as shown in the diagrams. When the strip

���A Diagram Showing Dimensions and Component Parts of Shunt

is firmly fastened, file off the stray solder which may have remained on the brass blocks. Cut a block of wood 6^4 ins. by 2<}4 ins. by iJs ins., plane it until smooth, and then sandpaper and shellac it. When it is dr\-, screw the brass blocks on it in the right place, according to the dimensions given in the accom- panying diagrams.

After it is securely fastened, connect two ammeters with two lo-amp. shunts, one a standard and the other the one described above, in series with a D. C. source. Pass 10 amperes through the meter with the standard shunt ; and note the reading on the other ammeter. In most cases it will read a little low, because the calculations for the width of the German silver strip may be a little inaccurate. Eile the shunt until both ammeters read the same.

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