ELECTRICAL ENGINEERING 359 



always work with one standard value of either direct or alternating current the slide wire 

 can be graduated to show voltages directly from either alternating or continuous currents. 

 By the use of an appropriate vibration galvanometer of the Tinsley type the Drysdale 

 potentiometer may be used for measuring alternating voltages even up to a frequency of 

 1,000 p. p. s. It is therefore extremely useful in telephonic measurements. By the use 

 of low resistance inductionless shunts it can measure large alternating currents- 



Another recent alternating current instrument of considerable utility is the iron 

 cored wattmeter of Dr. Sumpner. 



In this instrument an electromagnet of laminated iron is wound over with coils of thick 

 wire of low resistance. These coils are connected as a shunt across the terminals of the 

 appliance in which the power is to be measured. The field of the magnet is therefore pro- 

 portional to the time integral of the voltage when the -terminals are connected to an alter- 

 nating current supply. In the field of this magnet moves a pivotted coil with index needle, 

 the coil taking current from an air core transformer the primary circuit of which is in series 

 with the power absorbing circuit. Hence the secondary current in the coil varies as the 

 time rate of change of the main current. The coil is constrained by a controlling spring and 

 its deflections are then proportional to the mean value of the field of the magnet and the 

 current in the coil. When the frequency of these is the same this mean product is propor- 

 tional to the mean product of the voltage drop across the terminals and the current in the 

 power absorbing circuit and therefore to the power taken up by it. Hence the instrument 

 can be graduated to read power directly and can be arranged to read very small powers. 



Another recently devised arrangement for alternating current measurement is the 

 capacity bridge of Fleming and Dyke, by means of which the capacity of condensers 

 having considerable leakance can be measured and the leakance or conductivity deter- 

 mined at the same time for alternating currents. 



It consists of three condensers connected like the arms of a Wheatstone's Bridge. Two 

 of these condensers (C 3 ,C 4 ) are simple adjustable air condensers and the third (C 2 ) has in 

 series with it a variable resistance R.2. The fourth arm of the bridge consists of the leaky 

 condenser to be measured. In the bridge circuit a telephone is inserted and the conjugate 

 points are connected to an alternator giving a simple sine curve electromotive force. The 

 adjustment consists in varying the capacities and resistance until silence is produced in the 

 telephone. The capacity C of the condenser under test is then given by the expression 



C C* T 



C'= L 3 , 2 , , and its conductivity S by the expression S = pCx, where # = pC2R.2, p = 



-~ 



2-n-n and n is the frequency. By the use of this appliance the above named investigators 

 have examined the conductivity of numerous dielectrics for alternating currents of telephonic 

 frequency and proved that the conductivity is in general greater than the direct current 

 conductivity and is a linear function of the frequency n of the form S = A + Bn. 



2. ELECTROCHEMISTRY AND ELECTROMETALLURGY 



The applications of the thermal and chemical properties of an electric current in 

 industrial manufacture have become very extensive of late. Many products of great 

 commercial value can now be prepared electrically in virtue of the high temperatures 

 attainable which could not otherwise be produced. A good example is the substance 

 called carborundum, which is a carbide of silicon first prepared by E. G. Acheson in 

 1891. It is extremely hard and now largely used as a substitute for emery in grinding 

 wheels and polishing. It is produced at Niagara Falls by passing a current through a 

 mixture of sand coke and sawdust which is thereby raised to a temperature of 1950 C. 

 The chemical reaction is, SiC>2+3C= aCO+SiC. The carbon monoxide escapes and is 

 burnt outside the furnace. More than 3,000 tons of this are made annually at Niagara. 

 Acheson also prepares in the same manner graphite from coal (anthracite) an$ various 

 lubricating materials are in turn made from this graphite. All forms of carbon pass into 

 graphite at a sufficiently high temperature. Over 2,000 tons of graphite per annum 

 are now produced at Niagara Falls by this electrical process, and Acheson has dis- 

 covered methods of manufacturing not only the hard graphite suitable for electrodes, 

 crucibles and other objects, but a soft oleaginous graphite for making lubricants. 



Another extremely large electrothermal industry, in fact one of the largest, is the 

 manufacture of calcium carbide by electrically heating to a very high temperature lime 

 and pure coke or coal. The chemical reaction is CaO+3C = CaCa+CO. The calcium 



