Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Making an electric arc work from the mains

Demonstration

Carbon arcs once provided a common form of lighting. They make a surprisingly intense light source.

Apparatus and materials

Opaque, non-conducting screen

Insulating gloves

Connector box

Wooden stands, 2

Electric heater, small

Carbon rods, small or thick pencil leads, 2

Leads, plastic-covered crocodile clips attached at one end, the other end bare

Lens (+7 D)

Safety screen

Health & Safety and Technical notes


This must be an entirely teacher-based demonstration for safety reasons. 
 
Do it on a side bench. Use a safety screen to prevent anyone, including the teacher, from inadvertently touching a live contact at mains voltage. 
 
High voltage is in use so use shrouded leads. 
 
Light from the arc should be shielded from the eyes. Pencil leads will burn dramatically when first switched on since they have a high wax content.

Optional equipment:

Nichrome wire 24 SWG 
demonstration meter 300V AC 
demonstration meter 5A AC 
 
To measure the resistance of Nichrome you need: 

ammeter 0-1 A DC 
voltmeter 0-15 V DC 
power supply 0-12 V DC 

 

Procedure


a Make sure the connector box is disconnected from the mains; tighten the bare ends of the leads into the terminals inside. 

b As shown in the diagram, mount the carbon rods in the wooden stands so that they just touch. Attach the crocodile clips as close to the stands as possible. 

Apparatus set-up 
c Connect an electric fire into one of the sockets in the box. Make sure fire and box are safely placed. If the fire has a switch, check that it is on. 
 
d Put the opaque screen between the class and the arc to prevent direct light from the arc reaching students' eyes. (It can possibly lean against the safety screen.) Position the lens to project a real image of the arc onto a wall, for example. 
 
e Plug the box into the mains and switch on. Slowly pull apart the stands a small amount, touching only the base of a wooden stand supporting the neutral terminal; avoid looking at the arc. The arc is a flaring flame in the gap between the two white-hot carbon rods. 
 
f Adjust the lens to get a good image of the arc flame on a wall. 
 
NB Before touching the apparatus you must switch the power off; one of the conductors is likely to be live. 


Teaching notes


1 A small carbon arc with about 1 cm separation will run best at 5 amps. The carbon will be consumed too quickly at higher current. At lower currents the arc is likely to be unsteady. The arc itself needs about 70 V across it as a striking voltage, but then will run at about 50 volts. A resistor in series is necessary for stability and the electric fire serves this purpose. 

2 You could measure the current through the arc and potential difference across it. 
 
3 You could set a challenge for students: calculate how much 24 SWG Nichrome wire is needed in series with the arc to reduce its voltage to 50 – 70 V when operated from 230 volt AC mains. 
 
R = V / I = (230 – 50) / 5 = 180 / 5 = 36 Ω  

This is about the resistance of a 1 kW electric fire element. Either ask the students to use a multimeter (set on its resistance range) and find the resistance per metre of Nichrome, or give them the result. 
 
4 Alternatively, give students a length of Nichrome wire and let them measure its resistance using a 12 V power supply, ammeter and voltmeter. Calculate the resistivity of the wire ρ, using the relationship ρ = RA / l, where A is the cross sectional area of the wire and l is its length. Then calculate the length of the wire, x, needed for 36 Ω = ρx / A.  

This experiment was safety-checked in January 2007