TRANSACTIONS OF SECTION A. 575 



2. The logarithmic decrement of the receiver is of the same order of magnitude 

 as that of the vibrator. 



3. The chief component of the radiation and the period of the receiver may be 

 determined by a number of interference curves. 



4. The receiver could be used to analyse the radiation, where the oscillations 

 are but slightly damped. 



5. The error in determining the wave-length and the index of refraction need 

 not exceed 1 per cent. 



5. An Instrument for Becording Rapidly Varying Potential Differences 

 and Currents.^ By W. Duddell. 



The methods and instruments generally employed for this purpose may be 

 divided into two classes, viz. ' contact or point methods ' and * continuous 

 methods.' 



This latter class may be subdivided according to the nature of the moving 

 part acted on by the varying current. 



The present instrument belongs to that division in which the moving part 

 consists of wires carrying the current to be measured, and in its present form was 

 first suggested by Blondel. 



The instrument consists essentially of a pair of phosphor bronze strips stretched 

 tight in a strong magnetic field, to the middle points of which a small mirror ia 

 fixed. 



The current flows up one strip and down the other, causing one to move forward 

 and the other to move back, and thus turning the mirror through a small angle. 

 The source of light used is an arc lamp and a system of lenses, the motion of the spot 

 being recorded on a falling photographic plate, or observed in a rotating mirror. 



The necessary damping is obtained by immersing the strips in oil and adjusting 

 the temperature until it is correct. 



In the instrument shown two pairs of strips and a fixed mirror were used, sO' 

 that both the current and P.D. curves, as well as the zero line, were traced on the 

 plate at the same time, thus giving, as well as the two curves, their phase ditl'erence 

 and the periodicity from the known velocity of the plate. 



The free periodic time of the strips and mirror is about ^^xr sec, and a current 

 of jJjj cm. amp. gives a deflection of r53 cm. at a screen distance of 136 cms. 



The chief advantages of the instrument are the low self-induction and resistance^ 

 as well as the critical damping. 



6. Report on Electrical Standards. See Reports, p. 207. 



On the Calculation of the Coefficient of Mutual Induction of a Circle 

 and a Co-axial Helix. By Professor J. Viriamu Jones, F.R.S. 



8. On a Determination of the Ohm made in Testing the Lorenz Apparatus 

 of the McGill University. By Professor W. E. Ayrton, F.R.S. , and 

 Professor J. Viriamu Jones, F.R.S. Ajjpendix to Electrical Standards 

 Report. — See Reports, p. 212. 



9. On the Relations between Arc Curves and Crater Ratios with Cored 

 Positive Carbons. By Hertua Ayrton. 



When an arc is burning between a solid negative carbon and a positive of given 

 meter, the P.D. between the carbons varies according as the positive carbon is 



' Published in the Electrician. Sept. 10, 18J)7. 



diameter, 

 cored or solid 



