472 Prof. T. R. Lyle on 



Obviously in making the comparison it is desirable to 

 place the moving brushes so that the deflexions obtained are 

 near the maximum ordinate of the current wave. 



Otherwise we might put a resistance i\ in the galvanometer 

 circuit when p x is in, and r 2 when p 2 is in, so as to obtain 

 equal deflexions ; then we would have that 



Mi _ n 



M 2 ~~ r 2 * 



Hence the coils s and p L may be finished and their ter- 

 minals permanently fixed, and, using the latter (equal de- 

 flexion) method a resistance r\ found to give a large deflexion 

 when p 1 is in circuit and the brushes placed so as to give 

 approximately the maximum ordinate of the current wave. 

 Now substitute r 2 obtained by the above relation, allowing 

 for galvanometer or other appreciable resistance that may be 

 in circuit, and switch p 2 on to the commutator and galvano- 

 meter. An assistant can increase or reduce the turns (or 

 fractions) until the same deflexion is obtained as was given 

 by pi with r x in circuit. Spare wire should be left in case 

 the further treatment of the coils causes any change in their 

 M ratio. 



As it is necessary, if high accuracy with the wave-tracer 

 be desired, that the M coils used should have high insulation, 

 the triad should. now be thoroughly dried in an oven to drive 

 off moisture from the cotton or silk covering and placed in 

 an iron dish that can be fitted with an air-tight lid that has 

 an exhausting tube through it. A layer of solid paraffin is 

 in the bottom of the dish, and on this the coil is placed and 

 weighted with a piece of metal. The lid is luted on with 

 melted paraffin, the air pumped out, and the dish then heated 

 so as to melt the paraffin it contains. The coil now sinks in 

 the melted paraffin, and as the air has been removed it 

 becomes thoroughly impregnated. We can thus obtain coils 

 of high insulation and permanent mutual inductance if, in 

 addition, proper attention is paid to the insulation of the 

 terminals. 



A final and careful adjustment of the ratio of M 2 to M 2 

 should now be made with the wave-tracer. 



The absolute values of M x and M 2 must also be known, 

 and they can easily be obtained by either of the methods 

 just described in terms of a known standard of mutual in- 

 ductance. An alternating current is sent through the primary 

 of the standard, and either p l or p 2 of the triad (the one 

 whose M is nearest in value to that of the standard) and the 



