Show standard electrical instruments of the state school of mines written for the review by dr J F merrill in view of the vast sums of money paid for electric lights and power in this intermountain country it sometimes seems strange that so little littie attention is paid to the accurate measurement of this electrical energy in general the measuring instruments used are of the cheapest sort and give at best only approximately correct results yet these results are accepted as accurate and are made the basis of commercial transactions aggregating in value hundreds of thousands of dollars annually now it would be to the interest of all concerned if these transactions could be based upon reliable measurements of the power sold when n this fact is generally some idea of what the facilities are may be obtained from the accompanying cuts fig I 1 gives a view of the standard electrical instruments possessed by the school nos 1 and 2 are standard resistance boxes and no 3 is a wheatstone bridge and resistance si box all being certified to as being accurate within 1 25 per cent nos 4 and 5 are weston semi portable laboratory standard direct current meters one being a triple scale ammeter and the other a triple scale voltmeter volt meter the latter being provided also with a multiplier of factor 2 the ammeter has ranges 0 to 20 0 to 50 and 0 to amps respectively and the voltmeter volt meter of 0 to 3 0 to 15 0 to and 0 to volts a siemens electrodynamometer electro dynamometer of double ran range langean gean an instrument that is used as a standard in measuring alternating currents no 13 shows two silver volta meters of the rayleigh form no 14 gives a view of two cases of carhart clark standard cells each base case containing two cells and a thermometer and last we have nos 15 and 16 showing two standard certified to by the national bureau of standards at washington no 15 being ohms carrying currents up to 60 amps and no 16 being 01 ohm designed to carry currents up to 10 amp now all the instruments shown in this cut are provided with certificates of accuracy furnished either by the manufacturer or the national bureau of standards those possessing the highest degree of accuracy being the instruments numbered 1 2 3 4 5 6 7 14 15 and 16 but weston portable standards numbered 8 9 and 10 F 1 17 P 17 j Z 16 7 aa xan wt N 3 alk biffi 4 AA 4 71 N M FIGURE 1 understood no doubt there will be established by law standardizing bureaus where commercial electrical instruments will have to be taken for calibration i e to be tested so that the amount of error in their indications may be determined at the present time there exists no such a bureau in utah it may not be generally known however that the electrical department part ment of the state school of mines possesses facilities for testing and callb calibrating rating all kinds of commercial and other electrical measuring instruments ammeters voltmeters volt meters watt meters etc and the public generally may get the benefit of these facilities by sending their instruments 1 to the department to be tested no 6 shows a lord kelvin composite balance no 7 being its resistance this is a standard instrument for both direct and alternating currents and may be used as an ammeter with a range of 0 to amps as a voltmeter volt voit meter with a range 0 to volts and as a wattmeter watt meter no 8 shows two weston standard portable b le watt meters the school also possesses three other ins instruments trumen ts like these each being of a different capacity no 9 shows one of severa several i weston standard A I 1 D C voltmeters and its multipliers by means of which it will measure up to volts the school also owns several weston pa portable ratable standard ammeters and voltmeters like th the ethree three shown by no 10 no 11 la 13 are sufficiently accurate for commercial testing they being the best portable standards made indeed all the instruments shown are the very best of their class and make up a set of working standards that are accurate and reliable and cannot be surpassed for general excellence it will be noticed that there are standards for both direct and alternating currents amperes volts and watts and since some can be used for either direct or alternating currents this fact is taken advantage of in order to compare the accuracy cy of one instrument with another by the process of comparison it becomes easy then to discover any error that m may aparise arise in the indications of any instrument due to any cause whatever A good method of testing the accuracy of a direct current voltmeter by means of stand C R R R R R j s sc Q K u C A S P connections FOX AMMETER calibration R FIGU i louici IM a 21 ard resistance boxes and standard cells is indicated by fig 2 B is a storage battery baft ery V the voltmeter to be tested rl and ra standard plug resistance boxes of at least ohms capacity each S C is a standard cell and G is a sensitive galvanometer the voltage of the storage battery may be changed the method consists in balancing the fall of potential over ra against the E M F of the standard cell or cells and then calculating the fall over rl and ra and comparing this with the voltmeter reading by suitably changing the number of cells in B and the values of rl and ra readings throughout the entire range of the voltmeter scale may be obtained if the E M F of the standard cell is B E the correct voltage at the terminals of the E eritz ERi R Rz voltmeter is v jg is ig the ua value vahie that is compared with the voltmeter indication there is a slight modification of this method for low reading voltmeters but it will not be given here the accuracy of the weston and kelvin voltmeters may thus be tested and these in turn be used for testing other voltmeters now if the kelvin instrument is correct for direct currents voltage it will be correct for alternating current voltage also next fig 3 indicates the connections for testing j the tk accuracy of an ammeter using a standard cell and two standard plug resistance boxes S R is a standard low resistance for carrying current and rl ra are standard resistance boxes of at least ohms capacity each B is a storage battery R is a rheostat A is the ammeter to be tested and ri C is a standard cell ceil first ma making king rl 41 and ra at least to ohms and keeping this sum constant throughout balance the fall of potential over ra against the known B M IF E of S C indicated by no deflection of the sensitive galvanometer G observing the corresponding value of ra next with a steady current through the ammeter balance the fall of potential over S R against the fall over a suitable number of ohms in ra call this number ra E F ra then I 1 in ua ra where rl is the value in ohms of the standard resistance b R and B E is the e m f of the standard cell T thus hus the weston semi portable standards or the kelvin balance etc may be coma compared with the standard cells and ana the standards of resistance further by means of the silver voltmeters and stands d resistance boxes the accuracy of the cells may b be tested it will be seen therefore that the accuracy of any one of these standards may be tested in terms of the others hence aken together they form a set of standards by means of which perfectly reliable calibrations brat ions of commercial instruments may be made besides the standards shown in fig 1 the electrical laboratory is provided with an excellent equipment of po portable ratable instruments ammeters voltmeters watt meters etc for electric testing fig 4 shows a case of ift 1 1 connections t ron MR g c HR ar 1 VOLTMETER vc LT METER C LI B ration 0 N J 1 i 1 i t 1 i 1 f t h i i R i ra B c L B i 1 1 1 h FIGURE 3 these instruments several different makers being represented in the collection besides the instruments shown there are several volt and current transformers and a marble panel switchboard provided with five of the best type of westinghouse instruments the instruments were all selected with reference to the purposes for which they were to be used these purposes will be made evident in a later article on the machinery of the electrical laboratory CEO p aa pig T 6 aft aft FIGURE 4 |