Van De Graaff Generators

Description

The "IEC" 250mm diameter Van De Graaff generator is a modern, compact, and robust instrument specially designed with all parts fully exposed for classroom use. This model is available either without (EM4134-101) or with remote speed control (EM4134-101R).

Specifications

• Output: EM4134-101 400kV under good conditions. Approx. 15cm spark gap.
• Discharge ball: 80mm diameter with insulated handle and earth terminal connection.
• Charging Belt: 55mm wide silicone rubber (Part No. PA4138-010S).
• Drive belt: Belt driven from a speed-controlled DC electric motor.

Note on Earthing

Most countries have an earthed mains power system, but some do not. It is important that the mains power outlet and mains cable being used has an earth connection so that the spark discharge current can pass to earth. Refer to sections explaining discharge currents in the earth wiring of buildings for details.

Component Assembly and Adjustment

Fit Top Pulley to Frame

At the top of the tube, lift the upper pulley with the belt around it and take the frame. Fit the two ends of the pulley shaft into the two empty holes. Squeeze the two legs together a few millimeters and push them into the plastic collar. Adjust the "comb" so its edge is almost touching the belt surface. The plastic socket has a magnet to retain the pin inside the ball.

Bottom Pulley Arrangement

The bottom pulley should run true and smoothly. The bottom "comb" should be adjusted so it is vertical and its upper edge is almost touching the belt surface. The 'comb' is the device that induces the charge on the belt surface after the belt contacts the surface of the plastic pulley. It is important that the comb is as close as possible to the belt without touching it.

Ball Fitting and Retaining System

To Refit The Large Ball

Mentally imagine the pin's position in space to engage it into the plastic socket. When engaged, a magnet inside the socket will pull the pin and hold the ball firmly in place.

Ball Retaining System

Inside the main ball, a steel pin is retained into a socket by magnetic pull. Remove the ball by firmly bumping it upwards to break the magnetic pull.

Earthing the Base of the Van De Graaff

This charge created and sparked between the balls passes from the dome to the metal plate in the base and down the earth wire of the mains cable to the building's earth system. This can cause a voltage spike in the earth wire on each discharge, potentially damaging other equipment. To avoid this, it is desirable to connect a wire from one of the earth terminals to another earthed part of the building (e.g., a sink, water pipe). Suitable earthing points include: any metallic part of the building's construction, a metal pillar or beam, a large metal object like a filing cabinet, or a large metal window frame. Alternatively, a wire can be run through a window to an external pipe, garden tap, or a spike driven into the soil. Be sure NOT to connect to GAS PIPES.

Principle of Operation

The rubber belt is driven by the plastic pulley. As the belt leaves contact with the plastic pulley, negative electrons jump from the belt to the earthed comb, causing the belt surface to become positively charged. The belt carries this charge to the dome. The inside surface of the belt touches the metal pulley, becoming the same potential as the dome. The positive charge across the belt's thickness is removed by the sharp points of the upper 'comb'. This charge adds to the dome's existing charge, increasing it over time. The outer surface of the dome acquires an increasing positive charge relative to earth. This 'charge pumping' effect continues until the voltage on the large dome is sufficient to cause a spark discharge between the dome and the discharge ball.

Operating Conditions

Best results are obtained in a dust-free atmosphere of low humidity with a clean and smooth dome, discharge ball, and insulating tube. The machine should be placed at least 1 meter from walls, light fittings, and plumbing, especially if these present sharp corners or edges. The discharge ball should be earthed by connecting to the Earth terminal on the base using the supplied flexible wire. The unit itself can be earthed to a large mass using the other terminal.

During prolonged operation, the discharge ball rod (with or without the earth cable attached) may be 'parked' by inserting it into the hole in the discharge ball support device mounted on the base. To alter the spark gap, adjust the 'parking' angle of the discharge ball.

The discharge ball's insulated rod may be held in the hand. To avoid sensations to the hand, the earthing cable should be attached to one of the earthing terminals on the base.

General Precautions (All Models)

• Approach the operating Van De Graaff machine with caution at all times. A spark discharge, although harmless, can be uncomfortable.
• Never come close to a Van De Graaff if you are wearing electronic equipment like a 'pacemaker' heart instrument.
• If the discharge ball is closer to the dome than the user, sparking will jump to the ball first. Stand behind the discharge ball and do not approach too close to the charged dome. If the earthed discharge ball touches the dome, the dome will be held at earth potential and will be safe to touch.
• After switching off the machine, always earth the large dome by touching it to the earthed discharge ball before touching it with your hand.
• Being in proximity to a Van De Graaff can cause your body to collect a static charge, especially if wearing insulating-soled shoes. A small discharge may be felt when touching a neutral object.
• Handle the large dome gently; dents are difficult to remove and can reduce operating efficiency if serious.
• Always hold the discharge ball support rod by the insulated handle provided. Always have the earth cable attached.
• Sunlight can cause deterioration of materials. Store the unit in a well-shaded place, away from unshaded windows.

How to Run the Machine

Turn on the power switch to the ON position and adjust to a medium speed. After a short time, sparking will begin, producing strong repetitive sparks. Depending on conditions, it may take several minutes to achieve strong charging. Adjust the spark gap and motor speed as required. If the discharge ball handle is connected to the earth terminal on the base, it can be held by its handle to draw larger sparks from the main ball.

Maintenance and Cleaning

Dirt and moisture cause poor performance. If very dirty, the charging belt may be wiped with a cloth moistened with alcohol (methylated spirits). Ensure it is dry before use. A very small amount of talc can be rubbed into the belt's inner surface to aid charging. Wipe off excess talc.

The plastic insulation tube can be cleaned with a soft cloth or, if necessary, warm water and detergent. Never use solvents on the tube or plastic base. Both pulleys must be free from dirt or rubber build-up and can be wiped with a cloth moistened with alcohol. Allow all components to dry.

A normal cleaning routine involves wiping the large dome, small discharge ball, and plastic tube surface with a soft lint-free cloth. Dirt or dust particles on the dome or discharge ball will degrade sparking performance. Keep the system clean. Cover the instrument when in storage.

Fitting and Removing the Main Dome

To remove the dome, simultaneously bump it gently upwards with both hands and lift it from its socket.

The Charging Belt

The silicone rubber belt has high contact potential with the lower pulley material. If a belt loses charging ability, applying a small amount of talc to the rubber surface can improve insulation, reduce slip, and enhance performance. Wipe off excess talc.

Moulded IEC silicone rubber belts have superseded older latex belts.

Removal and Replacement of the Charging Belt

Lift the upper dome from its socket and twist the upper and lower 'combs' away from the belt. Slide the belt off the lower pulley so it hangs loose in the plastic tube. Pull the metal 'U' frame from the upper plastic socket with the belt dangling.

If necessary, loosen the knurled screw holding the 'comb' support rod. Spread the legs of the 'U' frame slightly to remove the pulley shaft from the two holes in the frame. Slide a new belt onto the pulley, allowing it to dangle down the tube to be caught at the bottom and slid onto the bottom pulley. Refit the top pulley shaft to the holes in the 'U' frame. Re-tighten the small knurled screw. Squeeze the two legs together slightly to hold the top pulley firmly and slide them into the plastic collar. At both top and bottom pulleys, adjust the comb to be almost touching the belt.

Useful Observations

• Observe discharge rates, colours, and intensities at various spark gaps.
• Note the pull of attraction between balls as charge increases.
• In the dark, observe the corona discharge from the tip of the discharge ball support rod.
• If a person stands on an insulating sheet and places hands on the dome, their hair may stand on end as the generator charges. This is a popular, harmless trick.
• Check polarity and measure actual current flow with a 0-25 micro-amp meter. A unit performing well with high charge rates and rapid sparks will have a short circuit current of about 4 microamps or higher. Poor performance despite high current may indicate dirt/dust on the dome/ball or unwanted discharges. Poor current usually means the belt needs replacement or the belt/pulleys are dirty. Clean with a soft cloth dampened with methylated spirits.
• Observe retention of charge after the machine is switched off.
• Note the difference between discharge to a ball and discharge to a point.
• After switching off, touch the belt surface to feel residual charge.
• In a darkened room, observe leakage paths, discharges to dust, brush discharge from combs, different spark shapes, and the effect of bringing objects near the dome at high potential.
• Observe the behavior of a scrap of paper dropped between the balls.
• Reduce motor power (torque) and observe the belt slow down as dome charge increases. Work is done to transport charge from low to high potential.

Remote Speed Control

For users uncomfortable reaching towards a sparking machine, the "Remote Speed Control" cable plugs into a provided socket, allowing speed adjustment without being close to sparks. When removed, control reverts to the machine's knob (model EM4134-101R).

Option: Remote speed control

Model EM4134-101R features a special socket near the speed control knob. Plugging the Remote Speed Control into this socket allows speed adjustment via the cable's knob. The belt can be stopped before approaching the Van De Graaff.