Electricity for public schools and colleges . istance, fixed between two terminals Aand B ; ^^ is a standard cell; ^ is a cell of a smaller thatwe desire to measure ; G and G are delicate galvanometers toindicate when there are zero currents. Contact keys are not repre-sented. Note.—As an example we may state that in a certain instrument the wireA B has a resistance of about 53 ohms. It is about 53 metres long, and iscoiled in a screw-groove cut round an ebonite cylinder. Now the total fall of V due to the battery P will be distributedbetween the portions ARE and A Q B of the circuit, a
Electricity for public schools and colleges . istance, fixed between two terminals Aand B ; ^^ is a standard cell; ^ is a cell of a smaller thatwe desire to measure ; G and G are delicate galvanometers toindicate when there are zero currents. Contact keys are not repre-sented. Note.—As an example we may state that in a certain instrument the wireA B has a resistance of about 53 ohms. It is about 53 metres long, and iscoiled in a screw-groove cut round an ebonite cylinder. Now the total fall of V due to the battery P will be distributedbetween the portions ARE and A Q B of the circuit, according to their respective , by manipulating R,we can arrange the AV be-tween A and B until it equalsthe Cq of the standardcell ; this will be indicatedby the fact that when E^ = e^,then no current will passthrough G, since the twoequal and opposite opposed. Since there isno current through this cell, and so no polarisation, therefore eo ? - A remains constant, and therefore E is also constant. As the. CH. xiT. MEASUREMENT OF RESISTANCES 235 of P gradually alters by polarisation, we can so diminishthe resistance of R as to maintain G undeflected, or E^^ constant. All the above may be regarded as a contrivance for keepingthe two points A and B at a constant AV equal to the eoof the standard cell. And the uniform wire A B gives us by itsgraduations any fractional parts of e^ that we desire ; since, byOhms law, the AV, eo, falls uniformly from A to B. Now, if we arrange the cell e as shown, and if we get zero-deflexion of G when we make contact at Q, where A Q = « and B Q = /, it follows that then e = E or that a ?? ^ — ; 7 ^o) a + 0 since evidently E^ = -—^. E .?^ Q AB Thus we can measure e accurately when less than ^o- JVote.—It may make matters clearer if we point out that the positive polesof the battery P, and of the cells e^ and e, are all connected with the same endA of the wire A B ; and their negative poles are
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Keywords: ., bookcentury1800, bookdecade1890, bookpubl, booksubjectelectricity
