170 HENRY A. EOWLAND 



the others by Messrs. Warden, Muirhead and Clark of the same place. 

 But on careful comparison I found that Warden, Muirhead and Clark's 

 10-ohm standard was 1-00171 times that of Messrs. Elliott Bros. On 

 stating these facts to the two firms I met no response from the first 

 firm, but the second kindly undertook to make me a standard which 

 should be true by the standards in charge of Professor Maxwell at 

 Cambridge." At present I give the result of the comparison with 

 these standards, as well as some others, and also with a set of resistance 

 coils by Messrs. Elliott Bros. 



Commutators. No commutators except those having mercury con- 

 nections were used, and those in the circuit whose resistance was deter- 

 mined were so constructed as to offer no appreciable resistance. The 

 commutator by which the main current was reversed, could be operated 

 in a fraction of a second, so as to cause no delay in the reversal. 



Connecting wires. These were of No. 22 or No. 16 wire and were all 

 carefully twisted together. The insulation was tested and found to be 

 excellent. 



Inductor for damping. This has already been described in my first 

 paper on ' Magnetic Permeability,' and merely consisted of a small 

 horse-shoe magnet with a sliding coil, which was introduced into the 

 secondary circuit. By moving it back and forth, the induced current 

 could be used to stop the vibrations of the needle and make it stationary 

 at the zero point. This is necessary in the method where the first throw 

 of the galvanometer needle constitutes the observation, but in the 

 method of recoil it is not necessary to use it very often. I prefer the 

 method of the first throw as a general rule, but I have used both 

 methods. 



This method of damping will be found much more efficient than that 

 of the damping magnet as taught by Weber, and after practice a single 

 movement will often bring the needle exactly to rest at the zero point. 



Arrangement of apparatus. Two rooms on the ground floor of a 

 small building near the University were set aside for the experiment, 

 making a space 8 m. long by 3-7 m. wide. The plan of the arrange- 

 ment is seen at Fig. 1. The current from the battery, in the Univer- 

 sity, entered at A, the battery being eighteen one-gallon cells of a 

 chromate battery, arranged two abreast and eight for tension. The 



18 As this is nearly a year since, and as I cannot tell when the standard will arrive, 

 I now publish the results as so far obtained, hoping to make a more exact comparison 

 in future. 



