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. 



