ELECTROSTATIC MACHINES

are for producing positive and electricity separately in large Of these the Wimshurst machine is perhaps the best It consists e entially of two gla plates placed close together to rotate in opposite directions. to the outer surface of each plate. charge is given to one of the the plates are set rotating, and other strips become charged by from the charged strip. The are collected from the plates by combs and stored in metal knobs. well-known induction machines of electricity at rest. All sub when suitably rubbed, become and are capable of attracting bodies. | Experiment shows that two kinds of electrifi cation and negative) produced thus quantitie s at the same time, rubbed with silk becomes positvely d; sealing-wax or vulcanite with flannel or fur becomes electrified. Similarly electri« fied bodies repel, di imilarly electrified bodies attract, one another. Metallic substances and other conductors do not exhibit these phenomena when rubbed, unle held by a handle made of gla or other insulator. A body with a positive charge of electricity is at a higher potential than the earth; when eiectrically connected to earth, electricity will pa from it to the earth until the potentials are the same, which happens practically instantaneously. A negatively charged body is at a lower potential than the earth; when electric a lly connected, it receives electricity from the earth until the potentials are equalized. The earth’s potential is taken as zero. As in electro-kinetics, electricity pa es from a point at higher potential to one lower potential, and without difference at of potential there can be no electric flow. Every electrified body tends to discharge itself to earth or to a neighbouring conductor, hence the space around it is in a state of strain and constitu tes.an electrostatic field. If an insulated conductor be placed in the field due to a positively charged body, it becomes negatively charged on the side next the body, and positively charged on the side remote from it. This is electrostatic induction. The transmitting medium (in this case the air) is called the dielectric. With gla or mica dielectrics the inductive effect is increased. These substances are said to have a higher specific inductive capacity (S.1.C.) than air, whose 8.1.C. is taken as 1. For purposes of ‘electrostatic measurement, the unit charge of electric it y is defined as that quantity which, placed at a distance of 1 centimetre in air from a similar and equal quantity, repels it with a force of 1 dyne. The force of attraction or repulsion between two charged spheres is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. This force in air may be exactly calculated from the formula f=: where f is the force, Q —, where vV is the potential, Q the charge, r the capacity and r the radius. Lastly, a body is its charge, when its potential isl. ‘The fundamental equation connect-