Popular Science Monthly 



951 



Testing Bell Circuits with 

 Ordinary Dividers 



IN making tests on a bell circuit it is 

 usually necessary to remove the insula- 

 tion or make connection only where the 

 wire is bared; however, the method illus- 

 trated here can be employed without harm 

 to the insulation. An ordinary pair of 

 dividers is used, their points being thrust 



The points of the dividers are thrust into 

 the insulation until they touch the wire 



through the insulation until they touch the 

 wire within. When the divider-points are 

 removed the tiny holes they made will close 

 sufficiently so that the insulation will be as 

 perfect as before. — Edward R. Smith. 



Changing a Direct Current Bell 

 into an Alternating One 



'TX) change a direct current bell into an 

 A alternating one on which a step-down 

 transformer is used, make the connections 

 in such a manner that the two poles of the 

 magnet go directly to the binding posts 

 of the bell; next connect in series with a 

 32-candlepower carbon filament lamp or 

 the secondary of a transformer and adjust 

 the clapper spring. — Aurelio Sierra, Jr. 



How to Make a Solderless 

 Joint on Wire 



IT is difficult to make a good wire con- 

 nection when solder is unavailable, 

 however with the following method a very 

 good joint can be made. Scrape about 6 

 in. of the wire to be connected, making 

 sure that all rust or grease is thoroughly 

 removed, then twist the wires tightly 

 together. A piece of tinfoil about i in. 

 wide is then wrapped tightly around the 

 twist. Be careful not to tear it. Then 

 apply another layer or two of the foil and 

 wrap it over with tape, so that no corroding 

 substance or rain can enter the joint. Pull 

 the tape very tight so as to insure a 

 good connection between the tinfoil and the 

 wire. Then paint with asphaltum. This 

 method is only to be used in case of emer- 

 gency. — Alexander V. Bollerer. 



Precise Resistance Measurement by the 

 Voltmeter -Ammeter Method 



THIS article is intended for the every- 

 day shop man and the average 

 electrician. The measurement of resistance 

 can be made very precise by the method 

 given. 



Two important points are: To be sure 

 that the instruments read correctly to .1 

 of one per cent, and to take ten or more 

 readings for each measurement. The 

 resistance for each reading should be 

 computed and the average taken of the 

 several readings for the final value. 



The theory of this method depends upon 

 Ohm's law: E=IR. The accuracy of the 

 results depends upon the readings and the 

 manner in which the instruments are con- 

 nected with the resistance. A high resist- 

 ance can not be connected in the same way 

 as a low one. 



It is well to know whether to connect 

 the voltmeter across the resistance, or the 

 resistance and ammeter together. For this 

 reason the nature of the sample should be 

 studied before an attempt is made to 

 measure it. 



For example, let a voltmeter be connected 

 across a high resistance only. The ammeter 

 in the circuit will record the current 

 through both the resistance and the volt- 

 meter, as they are in series with the 

 ammeter. There will be a very small 

 current flowing. If the resistance to be 

 measured is greater than the voltmeter's 

 resistance xnore current will flow through 

 the voltmeter than through the resistance. 



1 



MAIN LINE5 



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MAIN LINES 



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Diagrams showing location of ammeter in 

 line for measuring resistance accuratdy 



In this case (problem i) the ammeter 

 should be connected in series with the 

 resistance. The voltage drop across the 

 ammeter is very small compared to that 

 in the resistance. The per cent of error 

 will be very small when measured. 



Now consider a low resistance with a large 

 current flowing (problem 2). First, notice 



