I am making a probe for a 20T magnet in a He3 system.I understand that the wires connecting everything have to be twisted otherwise a current will be induced.Why is that current induced if the wires are not twisted?-Thanks
The wires leading to your magnet form a closed loop,which is necessary of course since all electronic circuits must form such closed loops in order to function.The reason you’ll get an induced current in these wires is that your dealing with a magnetic field.That field passes through the closed loop of wire,creating what’s known as a”flux”,and how much flux there is depends on how strong the field is times how large the area of your wire loop is.
Flux=Magnetic Field*Area of Loop
The danger comes in while you’re changing the magnetic field.As you increase(or decrease)the strength of the field,you’re causing the amount of flux passing through the wire & cable loop to change,and this induces a electromotive force,E.M.F.,in the wire.Basically,a voltage develops around the loop that is proportional to the rate of change of the flux,and pointing opposite to the way the current is changing.
E.M.F.=-Change in Flux/Change in Time
(if you use the right units)
This voltage difference causes unwanted current to flow by Ohm’s law:
Current Induced=E.M.F./Resistance of the wire loop
The reasons that wire wrapping machine gets rid of this effect are;first,that it effectively decreases the area of the closed loops by breaking it up from one large loop into several smaller ones(the twists).This decrease in loop area decreases the amount of flux through the loop.And second,the E.M.F.s induced in each successive twist alternate sign(one positive,one negative),canceling each other out so that no current flows.
This same technique is used anywhere that stray magnetic fields may be a problem,like in experiments running near large power supplies.,or where electric lines carrying data need to traverse long distances,such as phone and ethernet lines.