138 



MAGNETS AND ELECTRICITY 



FIG. 127. 



Sal Ammoniac Liquid and 

 Dry Cells. 



place. Thus polarization is prevented. Daniell's cell is much used 

 for the operating of telegraph instruments (cf. 155). 



152. Sal Ammoniac Cells. Sal ammoniac cells (Fig. 127) are 

 used where a strong current is needed for a short time, as for ringing 



telephone and house bells. The liquid used 

 is a water solution of sal ammoniac, or am- 

 monium chloride (cf. 112). The conduc- 

 tors are a rod of zinc and a plate consisting 

 of carbon and manganese dioxide. 



Dry Cell. A "dry" cell is enclosed in 

 a zinc jar, which serves as the zinc plate. 

 The space between the zinc and the carbon 

 is filled with a paste consisting of sal am- 

 moniac, manganese dioxide, charcoal, and water. The cell is closed so 

 that the water cannot evaporate. 



153. Currents and Magnetism. As we learned in 

 150, we can test a circuit for the presence of a current 

 by bringing the free ends of the conducting wires in con- 

 tact with the tongue. Of course this will not do for very 

 weak or very strong currents. A second way of testing 

 for a current is to bring the wire of a closed circuit near 

 a magnetic needle or compass. The needle will be turned 

 from its N-S position. If the current is sufficiently 

 strong, and the bare conducting wire is dipped into iron 

 filings, some of the filings will cling to the wire. 



Both of the last two tests show that a wire carrying a 

 current is surrounded by a magnetic field: that it is a 

 magnet. 



154. Electromagnets. Since a conductor that is 

 carrying a current is surrounded by a magnetic field, we 



