Prior to him, all that was known of electricity and magnetism was what the ancients knew, nothing more than that the lodestone possessed magnetic properties and that amber and jet, when rubbed, would attract bits of paper or other substances of small specific gravity.

参考译文:在他之前，人们对电磁学的认知还停留在古代，只知道天然磁石拥有磁性，以及琥珀和黑玉在被摩擦时能够吸附纸片或者其它具有细小重量/比重的物体/物质。

句子语法结构分析如下:

状语 :Prior to him

主语: all

修饰主语定语从句 ：that was known of electricity and magnetism

系动词:was

表语从句:what the ancients knew

状语：nothing more than that the lodestone possesed magnetic properties and that amber and jet, when rubbed , would attract bits of paper or other substances of small specific gravity

重点单词及短语解析

(1) prior to 在…之前;

(2) be known of 对...了解

(3) electricity and magnetism 电磁学 

(4)what the ancient knew 古代人所了解的

(5)nothing more than 仅仅，只不过

(6)lodestone 天然磁石

(7)magnetic properties 磁性

(8)amber and jet 琥珀和黑玉/煤玉

(9) when (they were) rubbed 当他们相互 摩擦时

(10) rub 摩擦

(11)attract 吸引/吸附

(11) bits of paper 纸片

(12) other substances 其它物质

(13) of small specific gravity 具有小比重的