246 THE ELECTRIC TELEGRAPH. 



the drawing plate, and to magnetic matter in the insulating 

 covering. The impurity of the metal may be avoided by 

 taking galvanic copper, or, as this is rather brittle and 

 requires to be melted over and over again before it can 

 be drawn, by taking copper which, when tested by being 

 held in the neighbourhood of a delicate magnet, affords no 

 trace of any magnetic metal. The adhesion of iron to the 

 wire may be prevented by drawing it through holes in agate 

 plates, or, if these are not to be had, by letting the wire 

 drawn through ordinary steel dies be placed for a few hours 

 in a bath of cold muriatic acid before being covered with 

 silk. Professor Tyndall traced the magnetism of his coil to 

 the silk, and believed that the green dye used in colouring 

 contains some magnetic substance. When he substituted 

 bleached silk, he found the disturbance vanish. For our 

 part we have always found white silk injurious to the eyes 

 of the workmen employed in winding the coils, and prefer 

 green on that account. Mr. Vogel, of Berlin, whose wires 

 are perhaps the most uniformly drawn and the best covered 

 of any we have yet met with, has introduced the use of 

 aniline for dyeing the silk with which he covers his wires, 

 thus satisfactorily removing the last difficulty, as aniline is 

 totally free from any perceptible magnetic influence upon 

 the most delicate needle system. 



20. Sine and Tangent Galvanometer. A combination of 

 both principles in one instrument has been made by Siemens 

 and Halske. It is furnished with two separate coils of wire 

 on the same ring one of a few turns of thick wire, the other 

 of many turns of thin wire. Two magnetic needles are also 

 used with this instrument : that for tangent readings is 

 short, and attached to a long brass or aluminium pointer ; 

 that for sine readings is longer, and attached to a similar 

 pointer. 



The ring round which the wire is coiled is supported by 

 a circular plate, carrying in its centre the compass-box; 

 and is turnable in a graduated metal ring, for the purpose 

 of reading off the angles through which the coil is turned 

 for sine readings. When the instrument is used as tangent 



