« PreviousContinue »
The spaces marked INF. are not bridged by a coil, and, in consequence, when the plugs are removed from these positions, an infinite resistance is introduced into the cir
cuit, or, in other words, its continuity is totally severed, a condition of things necessary to certain tests, as will be shown later.
Large-Size Wheatstone Bridge, by Nalder Bros. The accompanying illustration depicts what is known as the Post Office pattern of Wheatstone bridge, the only difference between it and the foregoing being that it is slightly smaller, and therefore more compact and portable, and also that two contact keys are added, one in the battery and the other in the galvanometer circuit.
Some of the latter-day bridges are made up in “Dial” form, as shown in the accompanying illustration, the proportional arms being in a straight line, as usual, but the
adjustable arm is divided up between four circular sets of blocks, representing units, tens, hundreds, and thousands respectively, contiguous blocks round the circumference
Post-office Pattern Wheatstone Bridge, by Nalder Bros. being connected to the extremities of the coils in numerical order, and the plugs connecting them severally to the centre piece, which, in turn, is connected to the outer extremity of the first coil in the next set, and so on.
“Dial” Pattern Bjidge, by Nalder.
The advantages of this form of bridge over the preceding pattern lie in the smaller number of plugs (one to each dial), and consequent reduction of the number of movements necessary in manipulating the bridge, together with the reduced risk of bad contacts owing to loose plugs. It also possesses attendant disadvantages, in that it is not so readily cleaned, and is, in consequence, more likely to harbour dirt than the original pattern; despite this fact, it has been widely adopted, and it is questionable whether this disadvantage is not outweighed by the advantages enumerated above.
Plan of Dr. Fleming's Improved Form of Wheatstone
Bridge. (Arranged in “Dial” form.) a Proportional arms. b, c, and d Adjustable arm. e Battery terminals. f Galvanometer terminals. 9 Line, or (x) terminals.
Catches for fastening lid of bridge. j Ebonite knobs for lifting out lid. k Ebonite handle to automatic spring revolving dust
plates. į Galvanometer key. m Battery key. n Plug
A very useful form of bridge for workshop use has latterly been put on the market by the Telegraph Manufacturing Company, of Helsby. It was designed by Dr. J. A.
Fleming, and is depicted in plan by Fig. 15. It has several novel points; the coils are of manganin wire wound non-inductively on a composite "former," consisting of two semi-cylinders of copper, separated by a centre strip of ebonite. The semi-cylinders are connected to the blocks, which are arranged in “dial ” form, by lugs, and no metal work, with the exception of six convenient terminals, is exposed on the exterior of the instrument. The plugs resemble brad-awls, being provided with hard wooden handles, as illustrated; they are normally placed in a rack fixed to the side of the containing box, and pass through holes in the wooden lid before reaching the metallic blocks. When the bridge is out of use it is rendered dust proof by a system of revolving spring discs of insulating fibre, which close the apertures, and thus prevent the entrance of dust and dirt. The shanks of the plugs are grooved at a point just below the upper surface of the blocks when they are in position; this grooving effectually prevents "shouldering,' a fault to which the old form of plugs were especially liable when they became worn to any extent. The proportional arms are also arranged in dial form, and a tablet with the inscriptions "multiply by” and “divide by," as applying to the resistance in circuit therein is affixed to the centre, and constitutes a great saving in memory, besides being conducive to speedy working. Two keys, in the form of press buttons, for galvanometer and battery circuits respectively, complete a very useful form of bridge.
The Wheatstone principle is also applied to yet another type of instrument, which will be found very useful in subsequent tests, to wit, the slide wire, or metre bridge.
It consists, Fig. 16, of a stout wire A B, exactly one
metre in length, composed of platino-iridium, or any other metal with a similarly low temperature co-efficient, stretched taut between two stout supporting straps, a and b, of copper, to which it is soldered or brazed.
d is a third copper strap mounted on the base of the instrument. Terminals are provided, as shown ; C is a contact slider with a knife-edge contact piece mounted on a spring plunger, which normally maintains it out of contact with the wire A B, but allows it to make contact with the latter at one point only when depressed. It is provided with an index finger exactly in a line with the point of contact, which indicates on a metre scale suitably divided and placed behind the slide wire as shown. 1 and 2 are a standard coil of adjustable resistance, and the unknown quantity to be measured respectively. The mode of using the instrument will be readily understood; the galvanometer is connected between C and d, and the battery to a and b; the slider C is then moved until, when the contact plunger is depressed, no current flows through the galvanometer. The same proportion then holds as before, viz., 1:A C: :2:C B. The wire A B having been carefully standardised in respect of homogeneity of resistance throughout its entire length, this quantity will be directly proportional to the respective lengths A, C, CB, as indicated on the attached scale, the readings on which in degrees can therefore be utilised in working out the above proportion.
Standards.—Electrical measurements, pure and simple, consist, like other measurements, in a comparison of the object to be measured or tested with certain known standards, which comparison determines what fraction or multiple of a certain predetermined unit or standard the said object contains, in much the same manner as that in which, when weighing a certain object, we determine how many pounds, ounces, grains, etc., would produce a position of equilibrium in the balance-beam or other weighing apparatus employed for the purpose given certain standard pounds, ounces, grains, etc., in the opposite scale pan.
The electrical quantities with which we are chiefly concerned in the pages to follow are :
(1) Electromotive Force, or Difference of Potential. (2) Current. (3) Resistance. (4) Capacity.