Page:Popular Science Monthly Volume 90.djvu/967

 Popular Science Monthly

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��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.

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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

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