PAPERS ON PHYSICS 287 



individual work was neat and perfect. At other times 

 practical electric burglar alarms for windows were worked 

 out and some students installed them in their homes. 



Electric dry cells are easily made, though they may be 

 weaker than boughten ones, by taking apart old dry cells 

 and using the zinc container and carbon stick, and pack- 

 ing a new pasty mixture of chemicals around the carbon 

 rod in this container. This gives the student an intimate 

 knowledge of the construction of such cells. A strong wet 

 cell, which students enjoy making and using, can be con- 

 structed with a cup of water in an eight ounce bottle, a tea- 

 spoonful of potassium bichromate and a tablespoonful of 

 sulphuric acid. Carbon and zinc rods, with wire attached, 

 complete the cell. 



With such cells a usuable medical battery, or shock coil, 

 can be constructed, using a simple switch, a buzzer for 

 make and break, and two pieces of coiled metal attached to 

 wires to hold in the hands. 



A miniature home lighting system is a good project. It 

 is made by mounting several sets of lamp sockets on short 

 boards, each set representing a separate circuit in the 

 house, and these all connected to the main line with switch 

 and fuse plugs. If a watt-hour-meter is at hand the cost 

 of using each lamp or circuit may be ascertained by the 

 students. 



Again, the students may mount more lamp sockets on a 

 board, having adjustable connections or switches between 

 them so that the sockets may be cut out or cut in at will, 

 and thus a handy lamp-bank is ready for a number of ex- 

 periments. 



Since students occasionally burn out valuable instru- 

 ments by using wrong connections, they ought to equip 

 themselves with a small fool-proof switchboard which tells 

 at once whether their connections are right or wrong. Hav- 

 ing learned the right connections to make with this small 

 switchboard, practically all danger of burning out valuable 

 instruments is avoided. 



After electromagnets have been made and tested our stu- 

 dents have put them to practical use by making simple tele- 



