No. XIV.] APPENDIX. 223 



For my own part, I should use the second plan, or the plan of 

 magnetizing by induction, to render the needle magnetic. For this 

 purpose, I have employed a temporary electro-magnet, which I magnetized 

 by the voltaic battery; and you will find that, by keeping the part 

 affected as close as possible to the instrument, for about half an hour, 

 you will sufficiently obtain the desired object. 



The electro-magnet might be made of the horse- shoe form, if we 

 knew the direction of the object ; but, in that case, we should not require 

 its use at all, as the proof of the existence of the needle is our only aim. 

 I have used the horse-shoe magnet, but should prefer in most cases an 

 electro-magnet like this (fig. 12), made for me by Messrs. Home, of 

 Newgate Street, which is made of a simple straight bar of soft iron, 

 wound round with wire. You will perceive by the diagram that the iron 

 has a plate of brass (B) fixed on both ends 

 to retain the wire (w) in situ; and you 

 may also perceive that the two ends of 

 the wire are attached to binding screws (s). 

 Your chemical lecturer has, doubtlessly, 

 made you aware that the magnetic effect, 

 cceteris paribus, is proportionate to the 

 power of the battery ; accordingly, you FlG - 12 - Electro-Magnet. 



must select a voltaic combination suitable 



for the desired object. You might use a Cruikshanks' battery, made 

 of alternate pieces of wire and copper soldered together. You might 

 use one of the old "Woollaston batteries, made of a plate of copper, 

 surrounding a plate of zinc. You might employ one or more Daniell's 

 batteries, which consist of an outer copper cylinder with a solution of 

 sulphate of copper, and an inner porous vessel containing zinc and dilute 

 acid. You might employ the battery invented by Mr. Grove : he uses 

 for his negative platinum, and in the inner porous cell he puts strong 

 nitric acid, and in the outer vessel with the zinc dilute sulphuric or muriatic 

 acid. It really is of no consequence whether you select the one or the 

 other battery for this particular purpose. I believe, however, that mine 

 is far more commonly used for the ordinary purposes of life. Of my 

 batteries you may use the triple or pot battery, which consists of a piece 

 of platinized silver, in the top of which is fixed a piece of wood to 

 prevent contact between the silver and the zinc. To the silver a binding 

 screw is fixed to connect it with any desired object ; a strip of zinc is 

 placed on each side of the wood, and both are held in their place by a 

 binding screw, sufficiently wide to embrace the wires and wood. You 

 may use the odds-and-ends form, which consists of a plate of platinized 

 silver for the negative pole, suspended in a vessel of acid, and fragments 

 of zinc and mercury, placed at the bottom of the vessel for the positive pole. 

 When you require considerable power, you will find the compound trough 

 battery very convenient for this purpose, formed of two plates of zinc, one 

 on each side the silver. The liquid generally adopted to excite the platinized 

 silver battery is a mixture of one part by measure of sulphuric acid, and 

 seven of water. The compound battery will magnetize a needle, in con- 

 junction with the electro-magnet at the distance of an inch, in the space 

 of two or three minutes. 



