What is the difference in pronouncing 'ß' and 'ss'?

'ß'- examples: Spaß; groß- the vowel before the ß, is pronounced longer  'ss'- examples: essen, Kasse, Russland, vergessen, besser- The vowel before the 'ss' is pronounced shorter It is also known that Diphthong (ei, au, ie, eu) are prounced longer(e.g. außerdem, genießen)  Beware: It is important to note that these sounds may be similar to already existing letters such as 's'. Hence this rule may not help to choose between 's' and 'ss' and/or ß.These rules are mainly applied in Germany. In Switzerland, these may vary.