(2)嘉塞尔购买力平价方程式
嘉塞尔说，两国的汇率，为两国货币对于各该国内货物的购买力相除之商所决定，是为两国间真正汇兑平价。他提议叫做购买力平价。
“According to the theory of international exchanges，which I have tried to develop during the course of war，the rate of exchange between two countries is primarily determined by the quotient between the internal purchasing power against goods of the money of each country...At every moment the real parity between two countries is represented by this quotient between the purchasing power of the money in the one country and the other.I propose to call this parity‘the purchasing power parity’.”[19]
如果两国中的一国或两国，采取货币膨胀政策，而其程度不同，例如甲国为320%，乙国为240%，则两国间的新的正常汇率应为旧汇率的3/4(正如那时的英美两国)。因此，得一规则，如两国货币膨胀，新汇率应为旧汇率乘两国膨胀系数之积。
“But as soon as an inflation takes place in the money of A，and the purchasing power of this money is，therefore，diminished，the value of Amoney in Bmust necessarily be reduced in the same proportion.And if the B-money is inflated and its purchasing power is lowered，the valuation of A-money in B will clearly increase in the same proportion.If，e.g.the inflation in Ahas been in the proportion of 320 to 100 and the inflation in B has been in the proportion of 240 to 100，the new rate of exchange will be three-fourth of the old rate(approximately the case of England and the United States).Hence the following rule:when two currencies have been inflated，the new normal rate of exchange will be equal to the old rate multiplied by the quotient between the degrees of inflation of both countries”.[20]
依此规则，我们得到
，即所谓新的正常英美汇率，亦即所谓英美购买力平价。又如某一时期，英国货币膨胀为200%，法国为600%，则新的正常英法汇率应为
=75fcs.[21]如果实际汇率(Actual rate)发生差异(Deviation)，则其原因应加以研究。
应用此学说时，应该注意两点:①两国物价指数基期应同为一年份，②两国物价指数的内容与其性质应约略相同，例如均用国际货物的批发物价指数。
嘉塞尔的学说也是数量说的一派，但是推广到国际金融上去，他的学说也受到许多人的批评。兹将贾克(Jack)的评语列下:①两国物价指数内容不易相同，所以比较不易正确。②两国中的一国所采保护贸易政策或许比对方国的政策不同，例如甲国对于乙国的货物输入限制较严，则乙国之物价必降落到购买力平价以下。此在欧战期间，美国货物输入瑞典大受限制，瑞货输美则不受限制，以至瑞币克乐拉(Krona)对美汇率远高于战前之汇兑平价(即Kr.3.75=$1)。在1917年，竟跌到Kr.2.55。③甲国采膨胀政策时，若乙国不如是，则甲国的外汇率必急速地上升。其上升程度必较其国内物价上升之程度更为迅速。例如1926年7月法国纸币流通增加11%，物价上升32%，而对英汇率平均上升53%。这是政治的不安定影响于各国之程度不同。④当汇价上升之时，人民易流于投机，争购外汇，以牟厚利，使外汇率之升降愈加剧烈。⑤资金的逃避亦于此时发生，更使外汇率发生动摇。以上三项都是心理作用，非购买力平价学说所能范围。此学说本由弊值不安定而生，但到真正安定之时所得购买力平价不足以为训。[22]
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