OF ARTS AND SCIENCES. 



309 



culty of heating bars of comparatively large diameter to a point beyond 

 that of dull red heat, this method was abandoned, and the following 

 was adopted. The testing apparatus consisted of an electro-magnet, 

 horseshoe in form, but the wire of which was placed at the bend of the 

 horseshoe, so that the electro-magnet was practically a straight one, with 

 a horseshoe-shaped core. Upon one of the limbs of the horseshoe the 

 induction coil was slipped, so as to still remain at right angles with the 

 electro-magnet. The soft iron bar or wire was then made the armature 

 of the electro-magnet. It was found that this arrangement was a very 

 sensitive one ; for any change iu the condition of the wire forming the 

 armature was immediately shown when the electro-magnet was excited, 

 and an induced current passed through the fine induction coil. This 

 method allowed us to experiment with wires, or bars of any suitable 

 diameter. For, as it will be shown later, the size of the armature had 

 very little effect upon the strength of the induced currents produced at 

 makins and breaking the current in the electro-magnet. We shall 

 speak of the horseshoe-shaped core and the armature as a magnetic 

 circuit, which of course is a mere convenient term. When the arma- 

 ture is applied to the poles of the horseshoe, and the electro-magnet is 

 excited, then such a circuit may be said to be closed. The bars or 

 wires were tested at a dull red heat, and also at white heat. The first 

 bar used was 2 mm. in diameter, and the following table shows the 

 results obtained. Six observations were taken, at intervals of one 

 minute apart. 



TABLE I. 



When the magnetic circuit was closed by the armature, the first 

 induced current which was produced by making the circuit of the 

 electro-magnet was greater than the succeeding ones. This was doubt- 

 less due to residual magnetism. We do not speak of the induced 

 current which resulted from breaking the circuit of the electro-magnet; 

 for this was equal to that produced by making the circuit. The mean 

 of the two renderings gave the correct result. 



