66 



KNOWLEDGE 



[March 1, 1899. 



instrument thus calibrated is known as a galvanometer 

 when its scale is marked in degrees, or some trigonometrical 

 function of degrees. But, to still further facilitate rapid 

 commercial measurement, the scale is usually arranged so 

 that it reads directly in amperes, in which case it is termed 

 an amperemeter, or an ammeter. It is not possible here 

 to refer to the very many electrical and mechanical details 

 which have to be considered, and to conditions which have 

 to be fulfilled, that the readings of such an instrument 

 may still be accurate although the magnetic field in which 

 it is used may vary within wide limits, and any spring 

 forming an essential part of its movement might be subject 

 to great changes in its resilience, while it will as a whole 

 undergo great changes of temperature. Further, errors 

 introduced by " hysteresis " must be eliminated, or, to be 

 more accurate, precautions should bo taken to prevent 

 their introduction. Hysteresis has been popularly defined 

 as " the past history " of the instrument (Prof. W. E. 

 Ayrton, if memory serve), an excellently bad pun, and an 

 excellently good definition in this connection, for it will 

 easily be realised that if an instrument, depending for its 

 action on electro-magnetism, be used first for the measure- 

 ment of a relatively large current, and immediately 

 afterwards to measure one of low strength, the residual 

 magnetism (a part of that magnetism previously induced 

 by the much larger current) will, if it be allowed to work 

 its wicked will, affect the second reading. 



Speaking broadly, then, the ammeter of commerce is an 

 instrument which depends for its action on the fact that a 

 mass of steel or iron will always tend to set itself in the 

 strongest part of a magnetic field. 



The measurement of electrical pressure, electromotive 

 force, or potential difference (as under varying circum- 

 stances the same " attribute" of electricity is called), is of 

 course referred to the volt. The volt is a derived unit ; 

 its derivation is not, however, far to seek. In order that 

 a current of one ampere may flow through a conductor 

 having a resistance of one ohm, the difference of potential 

 at the terminals of that conductor must be one volt. And 

 the ohm may be defined as the resistance of a column of 

 mercury, at the temperature of melting ice, 14'4521 

 grammes in mass, of a constant cross-sectional area, and 

 of the length of 106'3 centimetres. 



Since the volt is thus derived from the ampere and the 

 ohm, it is not surprising to find that so-called voltmeters 

 are really instruments to register the current strength 

 flowing through a wire of definite resistance. And since 

 we know that the potential difference at the terminals of 

 any conductor is the product of the resistance, in ohms, 

 and the current in amperes in that conductor (Ohm's 

 law), the instrument can be so constructed as to indicate 

 directly in volts. 



In one well-known type of direct reading voltmeter, the 

 increase in length of a stretched wire is actually taken as 

 a measure of the voltage, because the passage of current 

 causes heating, and the greater the current the greater the 

 heat and expansion produced. The wire usually employed 

 for " stringing " this instrument is of platinum-silver, and 

 very fine. This instrument (due, by the way, to Major 

 Cardew) has had a vast deal of thought expended upon it, 

 to bring it to its present accuracy. The rods supporting, or 

 forming part of, the frame upon which the wire is strung 

 are compounded of iron and brass, so that their temperature 

 co-efficient shall be equal to that of the platinum-silver, yet 

 inexpensive. If this precaution were not taken the pointer 

 would move through a certain angle on account of external 

 change of temperature. If one of these instruments be 

 used with the tube which encloses the wire placed vertically, 

 a certain amount of error is introduced, especially on the 



higher parts of the scale, as, the wire being considerably 

 heated when this is reached, air currents are set up in the 

 instrument and the needle will vibrate instead of taking 

 up a definite position. If the tube be horizontal this will 

 not occur. 



The Cardew voltmeter is not affected in the least by 

 external magnetism, and of course no temperature correc- 

 tion need be made, unless extreme accuracy is required, in 

 which case the constant will be negative, and of the order, 

 0-03 per cent, per degree Centigrade. 



There are many forms of voltmeter which may be 

 regarded as high resistance ammeters. In using all such 

 instruments the temperature correction is a most important 

 factor, because there must be very many turns of tine wire 

 (otherwise the potential difference to he measured would 

 he destroyed by the insertion of the instrument forming an 

 easy path through which the pressure could send a 

 current), and these many turns would, by the rise of 

 temperature, have their aggregate resistance proportionally 

 increased. 



This difficulty has been recently overcome by the 

 introduction of an alloy termed " Ja .Ja," which has a 

 negative temperature resistance co-efticient. By there- 

 fore arranging for a coil of a metal having a positive 

 co-efficient and an added resistance (of proper proportion) 

 of " Ja .Ja " — that Gilbertian name!! — tlae temperature 

 co-efficient of the instrument may be made absolutely 

 nothing. 



We have thus the essential parts of a direct current 

 direct reading wattmeter, an ammeter, a voltmeter, and 

 a clock. On alternating circuits, where the direction of 

 the flow may vary very many times a second, the condition 

 of things is different, and it must be remembered, in con- 

 sidering the wattmeter, that the product of mean current 

 by mean potential difference throughout a definite time 

 will not, as a rule, be equal to the mean value of the 

 energy expended. 



In practical forms of the wattmeter, the ammeter coil 

 usually has the voltmeter coil suspended within it, or a 

 small coil, connected up as a voltmeter, which, in order to 

 make up the required resistance, is joined in series with a 

 resistance ; if the latter is to have no electro-magnetic 

 effect, it is wound " back on itself," or double, so that the 

 inductive eflect of each half of this counterbalances, and is 

 counterbalanced by that of the other. 



As regards the addition of the clock, which converts the 

 wattmeter into what is popularly termed an " electricity 

 meter," this part of the apparatus is brought into gear in 

 various ways. The pendulum bob may be replaced by a 

 coil of wire, so that the electrical energy affects the 

 regulation, or the time-counting arrangement may be 

 motor driven. 



The want of space does not permit of the discussion of 

 any further forms of electric measuring instruments, though 

 we may in passmg point out that the public had no faith 

 in the registrations of electricity meters until some genius 

 was struck with the idea of making them look like gas 

 meters externally. 



It has been quite impossible here to refer to the entire 

 system of electrical units— indeed the Coulomb, Farad, 

 Joule, and Sechom are practically left out. Before leaving 

 the consideration of units it may be well to add that the 

 late Prof. Fleming Jenkin's " Report to the Royal Society 

 on the New Unit of Electrical Resistance," which, in con- 

 junction with British Association reports, 1862 and 1869, 

 were issued in book form, shows very plainly how complete 

 a system of mutually depending units has been arranged. 

 In this connection the story of the Board of Trade 

 standards would be well worth looking up. 



