248 Mr. Michael Donovan on certain Improtements in 



■witli a cotton-covered coil, although sometimes as sensible as most others, 

 often becomes singularlj' otherwise. On such occasions, it will not be affected 

 by a thermo-electric cun-ent generated by the heat of the fingers on two wires, 

 bismuth and antimony, soldered together, which at another time would move 

 the same needle 60' or 80°. I have not been able to connect these failures 

 with any particular states of the weather, although such states may be the cause, 

 acting jDcrhaps on the hygrometric properties of the cotton. Coils covered with 

 silk are not subject to this uncertainty in their action. 



As silver is said to be the best of all conductors of electricity, it might be 

 supposed that the wire of the coil should be of that metal. I made compara- 

 tive trials of silver and copper coils, each resembling the other in every respect 

 except the metal. I could perceive no decided difference in their effects, but 

 imagined the copper to have some little advantage, and therefore adopted it. 

 The difficulty of covering pure silver wire with silk is, as I am informed, great; 

 and this, along with the cost of the silver, made a difference in the total cost of 

 the galvanometer of more than £3. The advantage, if there be any, is certainly 

 not commensurate. 



I conceive that, in the generality of galvanometers, the coil and its frame are 

 too narrow: there is certainly an advantage in having them so broad that they 

 extend on each side nearly to the whole diameter of the graduated circle which 

 covers them. 



To conclude: — the improvements in the construction of galvanometers, 

 here suggested, may be summed up as follow: — 1. The addition of means, in- 

 dependent of the astatic needle (which may greatly err), for setting the instru- 

 ment in the magnetic meridian. 2. The close approximation of the needles to the 

 coil. 3. The removal of obstructions to the rotation of the needle. 4. The 

 means of inducing in the bars of the needle the least difference of polarity that 

 is consistent with their function. 5. A method of detecting and preventing 

 derangement of the needle arising from forces which cause in it a tendency 

 to stand transversely to its true position. 6. A construction of the needle 

 which renders available the operation of a strong or a weak directive force, as 

 may be required. 7. The introduction of a controlling graduated magnetic 

 power for increasing or diminishing the deflecting influence of voltaic forces on 

 the needles. 



