GUY'S AND ST THOMAS'S

MEDICAL AND DENTAL SCHOOL

 

Department of

Dental Materials Science

GUY'S HOSPITAL

LONDON BRIDGE

LONDON SE1 9RT

TELEPHONE: 071 9554549

FAX: 071 357 7563

 

Head of Departnent

D Brown MSc PhD CEng MIM

0719554551

M Sherriff BSc PhD MPRI ANCRT

071 9554550

R V Curtis BSc PhD DIC

071 955 4548

Professor N E Waters MSc PhD CPhys Finst p

071 9554548

 

DBROWN@UK.AC.LON.UMDS.PORTIA

DBrown@uk.ac.lon.umds.falstaff

 

12th December 1992

 

 

Dear Mr

 

Thank you for your letter regarding the voltage produced whenever metals are in contact.

 

Whenever such systems exist an electrical circuit also exists, and for a current to flow this circuit needs to be complete. Even on the microscopic scale such circuits always exist, and what we always need are two electrodes ( an anode and a cathode), an electrolyte (solution of ions) and an electrically conducting connection between the electrodes:

 

 

Under these conditions, provided the anode and the cathode are different metals, a voltage difference will exist, and when the circuit is complete a current will flow [as indicated on the meter}. When Galvani did the experiment his ‘meter' was the leg of the frog, which also acted as a source of electrolyte. The two connected metals combined to produce both the electrodes and the electrical connection.

 

When such a current flows in a Galvanic Cell the driving force has to come from somewhere, and the source of this force is the production of electrons at the anode, in the process we describe as corrosion. Hence:

Metal A - 1 electron = A+ [ an ion of metal A], or it may be

Metal B - 2 electrons = B++ [ an ion of metal B]

At the other electrode (the cathode) another reaction occurs, and this is usually associated with the decomposition of the electrolyte. For example,

2H+ [ions in water] + 2 electrons = H2 [hydrogen gas]

 

Only when both reactions can occur does a current flow, and the reason why batteries [for example] don't last for ever is that the paste electrolyte becomes saturated with the products of these galvanic reactions and the electrodes effectively become poisoned.

 

If, like me, you have amalgam fillings, then you are probably well aware of these galvanic currents when the foil from chocolate gets in contact with them, or we touch our amalgam fillings with the tang of a fork. My own mouth contains gold crowns as well as amalgams, but I am only made aware of their potential for creating galvanic cells when I complete the circuit with a metal artefact. Here at Guy's [as in all dental schools] we teach our student dentists to avoid placing gold and amalgam in our patients such that they are in permanent contact in saliva - one of the body's electrolytes.

 

Your sincerely,

Dr David Brown

Senior Lecturer in Dental Materials Science

 

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