218 REPORT— 1897. 



8. Defective insulation of parts of the circuit from one Effect would depend 



another. upon the position of 



the leaks. 



9. Permanent magnetic field at the apparatus. No effect, for the current 



through the field coil 

 was periodically re- 

 versed. 



As regards 4 and 7, special induction balances were constructed and 

 used by Mr. Mather to test the permeability of both the marble ring and 

 the phosphor bronze disc ; but, although a deviation from unity of one 

 part in fifteen thousand could have been detected in the permeability of 

 either, no such deviation was observed. 



As regards 5 and 8, careful tests were made every day of the 

 insulation resistance of the apparatus, and it was always found to be 

 greater than one thousand megohms. 



6. The insulation between the adjacent convolutions of wire could not 

 be measured when they were silk covered and buried in paraffin wax, 

 since a small leak between a pair of turns would not change the apparent 

 resistance of the copper coil by as much as the variation in temperature 

 of a fraction of one degree. We had, therefore, to content ourselves with 

 the precautions, previously described, which were taken to secure high 

 insulation in the winding of the coil. 



When the ring was wound with bare wire it was possible to roughly 

 compare the insulation resistance between pairs of convolutions by 

 sending a constant current through the coil and measuring, very 

 accurately, the P.D. between every adjacent pair of the 201 turns. This 

 we did several times, but it was a long and laborious task. 



When constructing a new Lorenz apparatus it will be well to consider 

 whether two separate helices should not be cut in the cylindrical surface 

 of the marble ring in which two independent bare wires would be bound, 

 a turn of the one being everywhere (except at the extreme ends) between 

 two turns of the other. The insulation resistance, therefore, between 

 the two windings would measure the insulation between the adjacent 

 turns, while in the ordinary use of the apparatus the two windings would 

 be joined in series so as to constitute a single coil. In this way it may 

 be possible to be more sure of the absence of 6 than by using paraffined 

 double silk covered wire, and at the same time, to entirely remove 1. 



The direction of our experimental result, which shows that the Board 

 of Trade ohm is between two and three parts in ten thousand larger than 

 the true ohm could not, however, arise from 1. Nor could it arise from 

 either 2 or 3, still many experiments were made to detect any evidence 

 of the effective diameter of the disc being larger than its true diameter, 

 as measured in the Whitworth machine. But no change in the pressure 

 of the circumferential brush-tubes, nor alteration in the shape of their 

 ends, &c., indicated that, with the brushes as we employed them, the 

 effective diameter of the disc differed from its true diameter. 



Our thanks are due to the three students whose names are given above 

 for much assistance in carrying out the long series of observations ; to Mr. 

 Harrison for bringing to bear, from time to time, the experience that he 

 had previously gained in the use of the Lorenz apparatus ; and we are 

 especially indebted to Mr. Mather for the suggestive aid which he rendered 

 us throughout the whole of the present investigation. 



