阅读《桃花源记》，根据要求回答问题。

    林尽水源，便得一山，山有小口，仿佛若有光。便舍船，从口入。初极狭，才通人。复行数十步，豁然开朗。土地平旷，屋舍俨然，有良田美池桑竹之属。阡陌交通，鸡犬相闻。其中往来种，男女衣着，悉如外人。黄发垂髫，并怡然自乐。

    见渔人，乃大惊，问所从来。具答之。便要还家，设酒杀鸡作食。村中闻有此人，咸来问讯。自云先世避秦时乱，率妻子邑人来此绝境，不复出焉，遂与外人间隔。问今是何世，乃不知有汉，无论魏晋。此人一一为具言所闻，皆叹惋。余人各复延至其家皆出酒食。停数日，辞去。此中人语云：“不足为外人道也。”

    既出，得其船，便扶向路，处处志之。及郡下，诣太守，说如此。太守即遣人随其往，寻向所志，遂迷，不复得路。

    南阳刘子骥，高尚士也，闻之，欣然规往。未果，寻病终，后遂无问津者。

阅读短文，完成下列问题

Dear Jenny,

   How are you? I am visiting my cousin Howard in Australia. It is hot in Hong Kong but it is cold in Australia in June. When it is summer in Hong Kong, it is winter in Australia. It is snowing heavily now. I like snow a lot and it is a white world outside. I will play with my cousin in the snow this afternoon. We want to make a snowman. I think it is very interesting!

   I like my cousin's house. It is big and there is a beautiful garden（花园）. We can play ball games. My cousin's neighbors are friendly but they cannot speak Chinese. I talk with them in English.

   See you next month.

 Tony

小电珠的伏安特性曲线如图所示（只画出了AB段），由图可知当小电珠两端电压由3V变为6V时，其灯丝电阻改变了____Ω．