208 



OUR PHYSICAL WORLD 



running parallel to the needle. The wire passed over the needle 

 in one direction, under it in the opposite direction. When even 

 a weak current is sent through the wire the needle is deflected. 

 By noting in which direction it swings one can tell the direction 

 of the current in the wire, and the amount of the swing tells 

 something of the strength of the current. This instrument is 

 called a galvanoscope (Fig. 84). 



Ampere made another important discovery, namely, that, if 

 currents of electricity are sent in the same direction, through two 



wires, set side by side and free to 

 move, the wires repel each other and 

 move apart. If the current is sent 

 in opposite directions the wires at- 

 tract each other and move together. 

 You can verify this for yourself in 

 this way. Fill a dish partly full of 

 dilute sulphuric acid made by pour- 

 ing the acid into the water. (The 

 acid is likely to spatter if you pour 

 water into it, and it burns badly.) 

 Fasten with tacks a strip of zinc on 

 one side of each of two good-sized 

 corks so that it sticks below the cork 

 an inch or two, and on the opposite 

 side of each cork tack a similar strip 

 FIG. 84.-A simple galvanoscope ofcO p per _ Wind good-sized insulated 



copper wire (No. 16) about a pencil to make a right-handed coil as 

 long as the diameter of the cork. Lay one of these coils on the 

 top of each cork and fasten the ends of the wire, one to the tack 

 that holds the zinc, the other to the tack that holds the copper. 

 Now float the corks on the sulphuric acid in the dish. A current 

 flows through each coil, for we have made a battery. The cur- 

 rent flows in the wire from the copper to the zinc. Bring the 

 corks close together with zinc strip facing zinc strip, and the 

 corks come together, for the currents flow in the adjacent coils in 



