ELECTRO-MAGNETIC EXPERIMENTS AND OBSERVATIONS. 467 



In- using this apparatus, we found it necessary to change the 

 acid frequently, otherwise the results were not so complete as 

 stated in the 3d and 4th series, and the action on the needle 

 appeared to be subject to capricious and sudden movements, 

 probably produced by the unequal action of the zinc and acid. 

 We' therefore constructed another apparatus, which combines 

 simplicity with cheapness. 



It consists of two single plates of zinc and copper, 4 inches 

 square, retained about f th inch apart by a slip of mahogany 

 rivetted to them, as in Fig. 2. This, while it preserves the 

 due position of the plates, affords a convenient support to the 

 connecting wires or helices, which can be instantaneously 

 changed without displacing the plates; for the ends of the 

 wires are merely inserted into holes which perforate the wood. 

 When experiments were to be made, the plates, thus connec- 

 ted, were placed in a shallow earthen-ware dish, containing 

 the acid liquor *. 



Our first experiments with this apparatus were made on 15th 

 November, and have been many times since repeated with the 

 same results. 



Experiments with the simple connecting-wire, Fig. 2. 



1. Zinc North. 



1 . Needle placed below the wire, deflected to the W. 



2. - above - - E. 



2. Zinc 



* The liquid which we found most convenient, is 1 part of nitric acid with 30 

 parts of water, and |- a part of sulphuric acid. The zinc plates were made accord- 

 ing to Messrs Silvesters' and Robson's patent, now in the hands of Messrs Phi- 

 lips, George and Co., about 2 a T inch in thickness ; the connecting wires and heli- 

 ces were of copper, or of brass wire, from J 5 to ^y inch in thickness ; the helices 

 most used had a diameter of about 2 inches ; and the needle employed was either 

 a small pocket compass, with an agate cap, or a naked needle, supported on a fine 

 point .fixed at right angles on a glass rod. 



